Introduction to Docker for .NET Developers

Containers package your app with everything it needs and run it the same way everywhere. No fiddling with machine-specific setups, no "works on my machine" jokes.

This guide keeps things hands-on: images vs containers, writing a Dockerfile,docker run basics, and a small docker-compose.yml to run your API plus a database.

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Images vs containers (quick mental model)

• Image → a read-only snapshot of your app and its dependencies (like a recipe).
• Container → a running instance of that image (like a prepared dish). You can start/stop/remove containers without changing the image.

Build an image once, run containers many times.

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A tiny .NET Minimal API to containerize

Create a folder HelloApi and drop this Program.cs inside:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "API is up");
app.MapGet("/hello/{name}", (string name) => new { message = $"Hello, {name}" });

app.Run();

.dockerignore (do this first)

Prevent large and irrelevant files from bloating your image:

bin/
obj/
*.user
*.suo
.vscode/
.idea/
**/*.Secrets.json

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Dockerfile (multi-stage build for .NET 9)

Create Dockerfile next to your .csproj:

# 1) Build stage
FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
WORKDIR /src

# copy csproj first to take advantage of layer caching
COPY HelloApi.csproj .
RUN dotnet restore

# copy the rest and publish
COPY . .
RUN dotnet publish -c Release -o /app /p:UseAppHost=false

# 2) Runtime stage
FROM mcr.microsoft.com/dotnet/aspnet:9.0 AS runtime
WORKDIR /app
EXPOSE 8080
ENV ASPNETCORE_URLS=http://+:8080

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

Why this layout?

• Multi-stage keeps the final image small (SDK isn’t in the runtime layer).
• Copying the .csproj before source allows restore caching until dependencies change.

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Build and run

# build the image (note the trailing dot)
docker build -t helloapi:dev .

# run a container mapping port 8080 in the container to 8080 on your machine
docker run --rm -p 8080:8080 helloapi:dev

Browse to http://localhost:8080/hello/Dotnet and you should see JSON.

Handy flags:

• --rm removes the container when it exits.
• -e KEY=VALUE sets environment variables (useful for connection strings, etc.).

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Common Dockerfile instructions (short list)

• FROM — base image (e.g., aspnet:9.0, sdk:9.0).
• WORKDIR — sets the working folder for subsequent commands.
• COPY — copies files into the image.
• RUN — executes a command at build time (e.g., dotnet publish).
• ENV — defines environment variables.
• EXPOSE — documents the port your app listens on.
• ENTRYPOINT / CMD — what runs when the container starts.

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Docker Compose: API + Postgres locally

Create docker-compose.yml at the solution root to run multiple containers together:

services:
  api:
    build:
      context: .
      dockerfile: Dockerfile
    image: helloapi:dev
    ports:
      - "8080:8080"
    environment:
      ASPNETCORE_URLS: http://+:8080
      ConnectionStrings__Default: Host=db;Port=5432;Database=hello;Username=postgres;Password=postgres
    depends_on:
      - db

  db:
    image: postgres:16-alpine
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: postgres
      POSTGRES_DB: hello
    volumes:
      - pgdata:/var/lib/postgresql/data
    ports:
      - "5432:5432"

volumes:
  pgdata:

Run everything:

docker compose up --build

• depends_on starts the database before the API (you should still add retry logic in your app).
• The pgdata volume keeps your database files between restarts.

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Dev workflow tips

• Rebuild fast: keep your Dockerfile steps cache-friendly (copy .csproj, restore, then copy source).
• Hot reload (local only): run dotnet watch on your host, and use a bind mount to share files into the container if you need the app to run inside Docker during dev:

  services:
    api:
      volumes:
        - ./:/src
      command: ["dotnet", "watch", "--project", "/src/HelloApi.csproj", "run", "--urls", "http://0.0.0.0:8080"]
  

• Prune old stuff: docker system prune -f removes stopped containers and dangling images. Careful when using this as this will delete images and containers.

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Quick troubleshooting

• Port already in use
  Change the host mapping: -p 9090:8080 and browse http://localhost:9090.

• File not found at runtime
  Ensure you COPY files into the runtime stage (or from the build stage). Check paths.

• Slow builds
  Make sure .dockerignore is in place and you copy the .csproj separately to benefit from restore caching.

• “Works outside Docker, fails inside”
  Check environment variables. Containers are isolated; they do not see your host’s env unless you pass them in (-e or environment: in Compose).

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A tiny checklist

• [ ] Use a multi-stage Dockerfile for .NET.
• [ ] Keep a tidy .dockerignore.
• [ ] Treat images as immutable—rebuild for changes.
• [ ] Map ports explicitly and pass env vars consciously.
• [ ] Use Compose when you have more than one container.
• [ ] Persist data with volumes in Compose.

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More!

# one-liners you’ll remember
docker build -t app:dev .
docker run --rm -p 8080:8080 app:dev
docker compose up --build
docker ps -a
docker logs <container>
docker exec -it <container> sh