fix(environ): ensure environ is loaded by systemd using a drop-in

[benfiedler]

PR progress checklist (to be filled in by reviewers)

• [ ] Changes to documentation are appropriate (or tick if not required)
• [ ] Changes to tests are appropriate (or tick if not required)
• [ ] Reviews completed

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What type of PR is this?

Primary type

• [ ] [build] Changes related to the build system
• [ ] [chore] Changes to the build process or auxiliary tools and libraries such as documentation generation
• [ ] [ci] Changes to the continuous integration configuration
• [ ] [feat] A new feature
• [x] [fix] A bug fix
• [ ] [perf] A code change that improves performance
• [ ] [refactor] A code change that neither fixes a bug nor adds a feature
• [ ] [revert] A change used to revert a previous commit
• [ ] [style] Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc.)

Secondary type

• [ ] [docs] Documentation changes
• [ ] [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

No.

Related issues and/or pull requests

https://github.com/saltstack-formulas/docker-formula/issues/317

Describe the changes you're proposing

Create+manage a systemd dropin file if 'environ' pillar is configured to ensure the environ file (/etc/default/docker) is being used by the docker service. A systemd daemon-reload will be run if salt detects changes in the override file, and the docker service will be restarted if salt detects changes in the environ file.

Pillar / config required to test the proposed changes

docker:
  pkg: 
    docker:
      daemon_config:
        insecure-registries:
          - my.repo.local:5000
      environ:
        - HTTP_PROXY=http://my.proxy.local:3128/
        - HTTPS_PROXY=http://my.proxy.local:3128/
        - NO_PROXY="localhost,127.0.0.1,.local"
      use_upstream:
        repo
  wanted:
    - docker

Debug log showing how the proposed changes work

Documentation checklist

• [ ] Updated the README (e.g. Available states).
• [ ] Updated pillar.example.

Testing checklist

• [ ] Included in Kitchen (i.e. under state_top).
• [ ] Covered by new/existing tests (e.g. InSpec, Serverspec, etc.).
• [ ] Updated the relevant test pillar.

Additional context