dotnet-containers

Imported from wshaddix/dotnet-skills (skills/dotnet-containers/SKILL.md). Install upstream with npx skills add wshaddix/dotnet-skills --skill dotnet-containers. Copyright stays with the author.

dotnet-containers

Best practices for containerizing .NET applications. Covers multi-stage Dockerfile patterns, the dotnet publish container image feature (.NET 8+), rootless container configuration, optimized layer caching, and container health checks.

Out of scope: DI container mechanics and service lifetimes -- see [skill:dotnet-csharp-dependency-injection]. Kubernetes deployment manifests and Docker Compose orchestration are covered in [skill:dotnet-container-deployment]. CI/CD pipeline integration for building and pushing images -- see [skill:dotnet-gha-publish] and [skill:dotnet-ado-publish]. Testing containerized applications -- see [skill:dotnet-integration-testing] for Testcontainers patterns.

Cross-references: [skill:dotnet-observability] for health check patterns, [skill:dotnet-container-deployment] for deploying containers to Kubernetes and local dev with Compose, [skill:dotnet-artifacts-output] for Dockerfile path adjustments when using centralized build output layout.

Multi-Stage Dockerfiles

Multi-stage builds separate the build environment from the runtime environment, producing minimal final images.

Standard Multi-Stage Pattern

# Stage 1: Build
FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
WORKDIR /src

# Copy project files first for layer caching
COPY ["src/MyApi/MyApi.csproj", "src/MyApi/"]
COPY ["src/MyApi.Core/MyApi.Core.csproj", "src/MyApi.Core/"]
COPY ["Directory.Build.props", "."]
COPY ["Directory.Packages.props", "."]
RUN dotnet restore "src/MyApi/MyApi.csproj"

# Copy everything else and build
COPY . .
WORKDIR "/src/src/MyApi"
RUN dotnet publish -c Release -o /app/publish --no-restore

# Stage 2: Runtime
FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime
WORKDIR /app
EXPOSE 8080

COPY --from=build /app/publish .
ENTRYPOINT ["dotnet", "MyApi.dll"]

Layer Caching Strategy

Order COPY instructions from least-frequently-changed to most-frequently-changed:

1. Project files and props -- change only when dependencies change
2. dotnet restore -- cached until project files change
3. Source code -- changes with every build
4. dotnet publish -- runs only when source or restore layer changes

# Good: restore layer is cached when only source changes
COPY ["src/MyApi/MyApi.csproj", "src/MyApi/"]
RUN dotnet restore
COPY . .
RUN dotnet publish

# Bad: restore runs on every source change
COPY . .
RUN dotnet restore
RUN dotnet publish

Solution-Level Restore

For multi-project solutions, copy all .csproj files and the solution file to enable a single restore:

FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
WORKDIR /src

# Copy solution and all project files for restore caching
COPY ["MyApp.sln", "."]
COPY ["Directory.Build.props", "."]
COPY ["Directory.Packages.props", "."]
COPY ["src/MyApi/MyApi.csproj", "src/MyApi/"]
COPY ["src/MyApi.Core/MyApi.Core.csproj", "src/MyApi.Core/"]
COPY ["src/MyApi.Infrastructure/MyApi.Infrastructure.csproj", "src/MyApi.Infrastructure/"]
RUN dotnet restore

COPY . .
RUN dotnet publish "src/MyApi/MyApi.csproj" -c Release -o /app/publish --no-restore

dotnet publish Container Images (.NET 8+)

Starting with .NET 8, dotnet publish can produce OCI container images directly without a Dockerfile. This uses the Microsoft.NET.Build.Containers SDK (included in the .NET SDK).

Basic Usage

# Publish as a container image to local Docker daemon
dotnet publish --os linux --arch x64 /t:PublishContainer

# Publish to a remote registry
dotnet publish --os linux --arch x64 /t:PublishContainer \
  -p:ContainerRegistry=ghcr.io \
  -p:ContainerRepository=myorg/myapi

MSBuild Configuration

Configure container properties in the .csproj:

<PropertyGroup>
  <ContainerBaseImage>mcr.microsoft.com/dotnet/aspnet:10.0</ContainerBaseImage>
  <ContainerImageName>myapi</ContainerImageName>
  <ContainerImageTag>$(Version)</ContainerImageTag>
</PropertyGroup>

<ItemGroup>
  <ContainerPort Include="8080" Type="tcp" />
</ItemGroup>

Advanced Configuration

<PropertyGroup>
  <!-- Use chiseled (distroless) base image for smaller attack surface -->
  <ContainerBaseImage>mcr.microsoft.com/dotnet/aspnet:10.0-noble-chiseled</ContainerBaseImage>

  <!-- Run as non-root user (default for chiseled images) -->
  <ContainerUser>app</ContainerUser>
</PropertyGroup>

<ItemGroup>
  <!-- Environment variables -->
  <ContainerEnvironmentVariable Include="ASPNETCORE_URLS" Value="http://+:8080" />
  <ContainerEnvironmentVariable Include="DOTNET_RUNNING_IN_CONTAINER" Value="true" />

  <!-- Labels -->
  <ContainerLabel Include="org.opencontainers.image.source" Value="https://github.com/myorg/myapi" />
</ItemGroup>

When to Use dotnet publish vs Dockerfile

Base Image Selection

Official .NET Container Images

Choosing a Base Image

• Default: Use aspnet for web apps, runtime for worker services
• Minimal footprint: Use chiseled variants (no shell, no root user, no package manager)
• Globalization needed: Use chiseled-extra if your app uses culture-specific formatting or time zones
• Self-contained or AOT: Use runtime-deps -- the runtime is bundled in your app
• Alpine: Smaller than Ubuntu but uses musl libc; test for compatibility with native dependencies

Rootless Containers

Running containers as non-root reduces the attack surface. .NET 8+ chiseled images run as non-root by default.

Non-Root with Standard Images

FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime
WORKDIR /app

# Create non-root user and switch to it
RUN adduser --disabled-password --gecos "" --uid 1001 appuser
USER appuser

COPY --from=build --chown=appuser:appuser /app/publish .
ENTRYPOINT ["dotnet", "MyApi.dll"]

Non-Root with Chiseled Images

Chiseled images include a pre-configured app user (UID 1654). No additional configuration needed:

FROM mcr.microsoft.com/dotnet/aspnet:10.0-noble-chiseled AS runtime
WORKDIR /app
# Already runs as non-root 'app' user (UID 1654)

COPY --from=build /app/publish .
ENTRYPOINT ["dotnet", "MyApi.dll"]

Port Configuration

Non-root users cannot bind to ports below 1024. ASP.NET Core defaults to port 8080 in containers (set via ASPNETCORE_HTTP_PORTS):

# Default in .NET 8+ container images -- no explicit config needed
# ASPNETCORE_HTTP_PORTS=8080

# If you need a different port:
ENV ASPNETCORE_HTTP_PORTS=5000
EXPOSE 5000

Container Health Checks

Health checks allow container runtimes to monitor application readiness. The application-level health check endpoints (see [skill:dotnet-observability]) are consumed by Docker and Kubernetes probes.

Docker HEALTHCHECK

FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime
WORKDIR /app

# Health check using curl (not available in chiseled images)
HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
  CMD curl -f http://localhost:8080/health/live || exit 1

COPY --from=build /app/publish .
ENTRYPOINT ["dotnet", "MyApi.dll"]

For chiseled images (no curl), use a dedicated health check binary or rely on orchestrator-level probes (Kubernetes httpGet, Docker Compose test):

FROM mcr.microsoft.com/dotnet/aspnet:10.0-noble-chiseled AS runtime
WORKDIR /app

# No HEALTHCHECK directive -- use orchestrator probes instead
# See [skill:dotnet-container-deployment] for Kubernetes probe configuration

COPY --from=build /app/publish .
ENTRYPOINT ["dotnet", "MyApi.dll"]

Health Check Endpoints

Register health check endpoints in your application (see [skill:dotnet-observability] for full guidance):

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddHealthChecks()
    .AddCheck("self", () => HealthCheckResult.Healthy(), tags: ["live"])
    .AddNpgSql(
        builder.Configuration.GetConnectionString("DefaultConnection")!,
        name: "database",
        tags: ["ready"]);

var app = builder.Build();

app.MapHealthChecks("/health/live", new HealthCheckOptions
{
    Predicate = check => check.Tags.Contains("live")
});

app.MapHealthChecks("/health/ready", new HealthCheckOptions
{
    Predicate = check => check.Tags.Contains("ready")
});

Container Optimization

.dockerignore

Always include a .dockerignore to exclude unnecessary files from the build context:

**/.git
**/.vs
**/.vscode
**/bin
**/obj
**/node_modules
**/*.user
**/*.suo
**/Dockerfile*
**/docker-compose*
**/.dockerignore
**/README.md
**/LICENSE

Globalization and Time Zones

If your app needs globalization support (culture-specific formatting, time zones), configure ICU:

# Option 1: Use the chiseled-extra image (includes ICU + tzdata)
FROM mcr.microsoft.com/dotnet/aspnet:10.0-noble-chiseled-extra

# Option 2: Disable globalization for smaller images (if not needed)
ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=true

Memory Limits

Configure .NET to respect container memory limits:

# .NET automatically detects container memory limits and adjusts GC heap size.
# Override only if needed:
ENV DOTNET_GCHeapHardLimit=0x10000000  # 256 MB hard limit

.NET automatically reads cgroup memory limits. The GC adjusts its heap size to stay within the container memory budget. Avoid setting DOTNET_GCHeapHardLimit unless you have a specific reason.

ReadOnlyRootFilesystem

For defense-in-depth, run with a read-only root filesystem. Ensure writable paths for temp files:

ENV DOTNET_EnableDiagnostics=0
# Or mount a tmpfs at /tmp for diagnostics support

Key Principles

• Use multi-stage builds -- keep build tools out of the final image
• Order COPY for layer caching -- project files and restore before source code
• Prefer chiseled images for production -- no shell, no root, minimal attack surface
• Use dotnet publish /t:PublishContainer for simple projects -- skip Dockerfile boilerplate
• Run as non-root -- use USER directive or chiseled images (non-root by default)
• Set health check endpoints -- enable orchestrators to monitor application state (see [skill:dotnet-observability])
• Include .dockerignore -- keep build context small and exclude secrets

Agent Gotchas

1. Do not use mcr.microsoft.com/dotnet/sdk as the final image -- SDK images are 800+ MB and include build tools. Always use aspnet, runtime, or runtime-deps for the final stage.
2. Do not hardcode image tags to a patch version (e.g., 10.0.1) -- use 10.0 to receive security patches. Pin to patch versions only if you have a specific compatibility requirement.
3. Do not use HEALTHCHECK with chiseled images -- chiseled images have no curl or shell. Use orchestrator-level probes (Kubernetes httpGet, Docker Compose test) instead.
4. Do not forget --no-restore on dotnet publish after a separate dotnet restore step -- without it, restore runs again and breaks layer caching.
5. Do not bind to ports below 1024 in non-root containers -- .NET defaults to port 8080 in container images. If you override ASPNETCORE_HTTP_PORTS, ensure the port is >= 1024.
6. Do not omit .dockerignore -- without it, the build context includes .git, bin/obj, and potentially secrets, increasing build time and image size.

References

• .NET container images
• Containerize a .NET app with dotnet publish
• .NET container image variants
• Chiseled Ubuntu containers for .NET
• ASP.NET Core health checks
• Docker multi-stage builds