IT | 2023-07-25 14:38:43

I have a project where I routinely build and rebuild containers between two repos, in which one of the docker build steps pulls the latest compiled code from the others repo. When doing this, the Docker cache gets in the way as it caches the published code. 

For example:

  • Project 1 publishes compiled code to blob storage
  • Project 2 pulls the compiled code and publishes a built container

Project 2's Dockerfile will look something like:

FROM ubuntu:22.04
RUN wget https://blob.core.windows.net/version-1.zip
RUN unzip /var/www/version-1.zip -d /var/www/

The issue is if I update the content of version-1.zip, Docker will cache this content in its build process and be out of date. 

I came across a great solution on stackoverflow: https://stackoverflow.com/questions/35134713/disable-cache-for-specific-run-commands

This solution doesn't work completely for me, as I am using docker-compose up commands, not docker-compose build. However, after a little trial and error, I have the below workflow working:

FROM ubuntu:22.04
RUN wget https://blob.core.windows.net/version-1.zip
RUN unzip /var/www/version-1.zip -d /var/www/

Run a build:

docker compose -f "docker-compose.yml" build --build-arg CACHEBUST=someuniquekey

Run an up:

docker compose -f "docker-compose.yml" up -d --build

This way the first run Docker build is cache busted using whatever unique key you want, and the second Docker up uses the newly compiled cache. NOTE: you can omit the last --build to not trigger a new cached build if you like. Now I can selectively bust out of the cache at a particular step, which in a long Dockerfile, can save heaps of time. I guess you could even put multiple args at strategic places along your Dockerfile and be able to trigger a bust where it makes most sense.