improved caching for dev docker

[epugh]

Description

Cache Ruby gems between Docker builds.

Motivation and Context

So SLLLOOOOOOOOWWWW to run bin/setup_docker. Very offputting. Make developers happy!

How Has This Been Tested?

On Master:
bin/setup_docker  11.46s user 7.59s system 5% cpu 5:39.34 totalOn 

On Branch First Time:
bin/setup_docker  11.47s user 7.75s system 3% cpu 9:02.38 total

On Branch After First Time:
bin/setup_docker  12.00s user 7.77s system 13% cpu 2:29.75 total

bin/setup_docker  11.61s user 7.77s system 14% cpu 2:15.32 total

bin/setup_docker  11.45s user 7.83s system 14% cpu 2:13.91 total

Types of changes

• [] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• [] My change requires a change to the documentation.
• [] I have updated the documentation accordingly.
• [] I have read the CONTRIBUTING document.
• [] I have added tests to cover my changes.
• All new and existing tests passed.