[PROPOSAL] Separate ARGS in Dockerfile for golang and python APPS_REPO / APPS_BRANCH

[nblair2]

Currently, the Dockerfile uses the pair of arguments APPS_REPO / APPS_BRANCH twice:

• first in lines 75-82 to build Go apps, and
• again in lines 155-160 to build python apps

I have some (admittedly unique) use cases where I need to grab different apps for golang vs python, so it would be nice to accomplish this with some simple docker build arguments, instead of some ham-fisted sed (which I am using now).

Would making this change break things for many users?