Explore elements of a Dockerfile

  • 3 minutes

A Dockerfile is a script that contains a series of instructions that are used to build a Docker image. Dockerfiles typically include the following information:

  • The base or parent image we use to create the new image
  • Commands to update the base OS and install other software
  • Build artifacts to include, such as a developed application
  • Services to expose, such a storage and network configuration
  • Command to run when the container is launched

Create a Dockerfile

The first step in creating a Dockerfile is choosing a base image that serves as the foundation for your application. For example, if you're building a .NET application, you might choose a Microsoft .NET image as your base.

# Use the .NET 6 runtime as a base image
FROM mcr.microsoft.com/dotnet/runtime:6.0

# Set the working directory to /app
WORKDIR /app

# Copy the contents of the published app to the container's /app directory
COPY bin/Release/net6.0/publish/ .

# Expose port 80 to the outside world
EXPOSE 80

# Set the command to run when the container starts
CMD ["dotnet", "MyApp.dll"]

Let's go through each line to see what it does:

  • FROM mcr.microsoft.com/dotnet/runtime:6.0: This command sets the base image to the .NET 6 runtime, which is needed to run .NET 6 apps.
  • WORKDIR /app: Sets the working directory to /app, which is where app files are copied.
  • COPY bin/Release/net6.0/publish/ .: Copies the contents of the published app to the container's /app directory. We assume that the .NET 6 app has already been built and published to the bin/Release/net6.0/publish directory.
  • EXPOSE 80: Exposes port 80, which is the default HTTP port, to the outside world. Change this line accordingly if your app listens on a different port.
  • CMD ["dotnet", "MyApp.dll"]: The command to run when the container starts. In this case, we're running the dotnet command with the name of our app's DLL file (MyApp.dll). Change this line to match your apps name and entry point.

We're not going to cover the Dockerfile file specification, visit the Dockerfile reference for more information. Each of these steps creates a cached container image as we build the final container image. These temporary images are layered on top of the previous and presented as single image once all steps complete.

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