Build functions in a temporary directory

Why

When building functions locally using Docker, gitlabktl copies runtime files into the user's working directory, and does not clean these files up when it is done. This is not a great experience, and it would be better to leave the user's working directory unmodified.

What

Copy the user's files to a temporary directory and build there. Ensure that this directory is deleted when the build is finished.

This is a follow-up to #28 (closed)