Simplify passing extra arguments to 'docker build'

With the docker/build component built on top of gitlab's auto-devops build, the correct method to pass custom arguments to the docker build command is via the AUTO_DEVOPS_BUILD_IMAGE_EXTRA_ARGS variable, see

https://git.ligo.org/help/topics/autodevops/customize.html#pass-arguments-to-docker-build

This isn't particularly intuitive.

Consider adding a new input called build_args (or similar) that sets that variable appropriately to simplify this and document it appropriately.