Add after_start and after_stop options to docker::run define

[jantman]

This pull request adds after_start and after_stop parameters to the docker::run define, working similarly to the existing before_start and before_stop options (i.e. these execute a command after the container is started or stopped).

PR includes unit tests. I haven't added these to the acceptance tests as the existing before_ variants weren't being tested there, but these are relatively simple template changes and we're currently using this code.

[codecov-io]

Codecov Report

Merging #580 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #580   +/-   ##
=======================================
  Coverage   28.02%   28.02%           
=======================================
  Files          19       19           
  Lines         685      685           
=======================================
  Hits          192      192           
  Misses        493      493

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[sheenaajay]

Thanks, @jantman for submitting the PR. Great work.