Dev 922 dockerize

[moseshll]

DEV-922 add tests and containerization for hathifiles-database

• Copy required files from ruby_github_actions_template repo.
• .gitignore update for SimpleCov and rspec_status.
• Appease Standardrb.
• Begin expanding test coverage.

Test coverage is still low, but what is there is non-trivial so I believe that acceptance criterion is met.

For Reviewers:
I would like a test of the clone, setup, test incantations. There is an awful lot of noise in the "files changed"-centric approach to reviewing.

Most of the changes fall into these categories:

• Mostly boilerplate Docker and CI additions.
• standardrb --fix results.
• Side effect of require "hathifiles_database/constants" instead of include: some added namespace verbosity. (This may be something that should be revisited in this PR.)
• Added specs

[mwarin]

lib/hathifiles_database/db/writer.rb and lib/hathifiles_database/db/connection.rb could do with a bit more test coverage.
Not that I think the numbers themselves are something to quibble over, but they appear to have important-looking methods uncovered.

[mwarin]

I see no reason to hold this back. Most of my comments are minor.

A PR like this could benefit from having the different kinds of changes grouped into different commits, so that there's one commit with the actual payload, one with the tests, one with the standardrb, and so on. Makes it easier to navigate the changes. Way easier said than done, though. Just something I want to think more about, and maybe I'll have something more useful to say.

APPROVE.