feat: aptible config:get cmd

[neurosnap]

https://app.shortcut.com/aptible/story/24724/aptible-config-get-to-retrieve-single-env-variable-value

aptible config:get APTIBLE_DOCKER_IMAGE --app app-ui-xxx-main
registry.io/yyy/xxx-ui:sbx-main

aptible config:get NOT_FOUND --app app-ui-xxx-main

I also added some docker files to make local development easier

[fancyremarker]

Tested, and LGTM. Behavior matches heroku config:get with default arguments; with APTIBLE_OUTPUT_FORMAT=json it's a little wonky but tolerable:

$ heroku config:get FOO --app test-frank
{
  "foo": "bar\nbaz\nslash\\"
}

$ bundle exec bin/aptible config:get FOO --app test-frank
{
  "foo": "bar\nbaz\nslash\\"
}

$ APTIBLE_OUTPUT_FORMAT=json bundle exec bin/aptible config:get FOO --app test-frank
[
  "{\n  \"foo\": \"bar\\nbaz\\nslash\\\\\"\n}"
]

[fancyremarker]

TY!

[fancyremarker]

Thanks for adding this (and the Dockerfile)! It is probably worth adding a quick note to the README about how to use.

[neurosnap]

@UserNotFound has a followup PR #331 where we can polish the local docker development -- where we can add a README update, I probably should have made a separate PR for it.

[fancyremarker]

FYI I needed to RUN rm -f Gemfile.lock here to avoid:

 => [5/6] RUN gem install bundler -v '< 2'                                                                        12.0s
 => ERROR [6/6] RUN bundle install                                                                                 0.2s
------                                                                                                                  
 > [6/6] RUN bundle install:
#0 0.227 /usr/local/lib/ruby/2.7.0/rubygems.rb:277:in `find_spec_for_exe': Could not find 'bundler' (2.1.4) required by your /app/Gemfile.lock. (Gem::GemNotFoundException)