Local plugin install

[mickmcgrath13]

Description

Introduces the functionality to use file:// for BitOps configs when building your own BitOps image to use locally-developed plugins.

It works by providing a plugins directory as a sibling directory to the Dockerfile (required for Docker build accessibility). When a plugins/ directory exists, the Docker ONBUILD directives will copy the contents into the container at /opt/bitops-local-plugins. From there, the BitOps-level bitops.config.yaml can reference the plugin like:

bitops:
  # ...other stuff
  plugins:    
    duplicate-environment:
      source: file:///opt/bitops-local-plugins/my-plugin
  deployments:
    my-plugin:
      plugin: my-plugin

Closes #308

Type of change

Please delete options that are not relevant.

• 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 not work as expected)
• Documentation update

How Has This Been Tested?

See the docs included in the PR. Tested with both local (file://) plugin references as well as URL based.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[arm4b]

Looked quickly, nice stuff! 💯

Will try the whole new experience end-to-end later.

[arm4b]

LGTM