CLI-1168: Improve Dockerfile

[danepowell]

This switches the base image to php-alpine, reducing build times, and adds an init script to make it easier to run.

An experimental build is on Dockerhub: https://hub.docker.com/repository/docker/acquia/cli

This pattern is based loosely on the Composer Docker image: https://github.com/composer/docker

If this catches on, we should make a few improvements:

• set up autobuilds on Dockerhub
• move the Docker files to their own repo, add a proper README
• update install docs on docs.acquia.com
• implement the xdg standards to allow moving the Acquia config directory out of /root.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (45b30b9) 91.83% compared to head (791347d) 91.83%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1618   +/-   ##
=========================================
  Coverage     91.83%   91.83%           
  Complexity     1816     1816           
=========================================
  Files           124      124           
  Lines          6505     6505           
=========================================
  Hits           5974     5974           
  Misses          531      531           

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