ci : reduce musa image size (#3277)

ci : reduce musa image size (#3277)

* ci : reduce musa image size

This commit contains an attempt to reduce the size of the musa Docker
image by copying only the necessary files from the build stage.

The motivation for this is that the CI runs sometimes fail with out of
memory errors. These seems to be able to pass for PRs, at least
sometimes but fail upon push to the master branch.

* ci : remove build time files instead of selective copying