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Building an image

To build an image we need a Dockerfile, as seen earlier. With it we generate our own images in an organized and reproducible way.

The command is:

docker build --tag image-name:0.1 .

Two notes:

  1. Docker sends the current directory (the . at the end of the command) as the build context. By default it looks for a file named Dockerfile in that directory; to use another one, pass --file path/to/Dockerfile.
  2. Since Docker 23 the build is done by BuildKit by default, which is faster, caches better and runs stages in parallel. docker build and docker image build are equivalent.

Example:

docker build --tag workshop/nginx:0.1 .

Run a container based on the freshly built image:

docker container run --detach --publish 46000:80 workshop/nginx:0.1

Go to http://localhost:46000

  • What happened?
  • Why does changing index.html on your machine not change anything on the page? (The image is a snapshot: the file was copied at build time. To see live changes, you need a bind mount, which we will see in Volumes.)

Building for multiple platforms

To produce images for more than one architecture (for example amd64 and arm64) and push them at once to a registry, use Buildx:

docker buildx build \
  --platform linux/amd64,linux/arm64 \
  --tag your-username/workshop-nginx:0.1 \
  --push .