Improve caching of docker builds

[hamishfagg]

This change makes a large portion of the docker build time (pip installing all our requirements) be able to be cached more often.

Cache misses were happening when almost any file changed in the repo, and most requirements would have to be installed again without being cached.

In this PR we instead:

• create a layer containing only the files required to run pip install . (requirements files, setup.py etc).
• Copy files from this layer and run pip install . and pip install .[<extras>] (this will now only miss if a requirements file or setup.py etc have changed)
• Now copy the whole repo into the container and run pip install . again - this will only install MindsDB itself
• Continue with the build as before

[mindsdb-devops]

see comments, minor layer name missing

[mindsdb-devops]

lgtm