Release fix

[davidslater]

Fixes #1531

This updates the build script to not require pip installation. We already require it to be run from the root of the armory repo, so it seems weird to require a pip install to simply import armory.

[lcadalzo]

lgtm, just one minor comment unrelated to this PR: we currently allow doing either a single build or all the builds. If we were to add nargs='+' to the parser.add_argument() call, this would allow for an arbitary number e.g.

python docker/build.py pytorch tf2

to build the tf2 and pytorch images. If we make this change, we'd need to modify the code below

else:
    frameworks = [args.framework]

since argparse would then already parse the input(s) as a list

[davidslater]

@lcadalzo regarding the nargs comment, I think maybe we can revisit that if we expand the number of docker images we create. However, for now, I would like to reserve the possibility of adding more non-keyword args without having to remove the list functionality.