Add Python to the PATH in the Dockerfile

[acwhite211]

Making life easier. Add /opt/specify7/ve/bin to the PATH in the Dockerfile. Run Python commands in the Specify 7 container without having to the full path to Python. I've had this on my local instance for a while, just spreading the love into the main branch.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)

Testing instructions

• Bash into the Specify 7 Container. docker exec -it specify7 bash
• Run a python command, like python manage.py showmigrations in the specify 7 container.
• Also run docker exec -it specify7 python showmigrations.