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One of my projects uses Playwright, and that involves a pretty big image. When you run the Playwright install, even if you only include one of the browsers, the size of that layer is about 800MB.

This was a quick investigation to see how I could control rebuilding image layers. Remember of course that once one layer needs to be rebuild, all the layers below that layer will also need rebuilding.

In this example I wanted to rebuild the RUN git clone... and below if I had changed the repo. To achieve that, I added the ARG CACHEBUST line above the RUN git clone.... The value of CACHEBUST is defined in the docker-compose.yml file and to force a rebuild, the value needs to be changed. This will cause that layer and all subsequent layers to be rebuilt.

The actual value for that ARG is stored as part of the Image and so if you haven't cleaned up your images and you set it to a value that you have used before, you might find you use an old cached version instead of building a new image.

Note that in order for this to work, you must include the --build option when running your docker-compose cmd e.g.:

docker-compose up --build -d

I also decided to get rid of the startup.sh file and call my real CMD directly in the Dockerfile as I was having issues with permissions and line endings.