Move container CI into a separate workflow

[collang]

• Container CI moved to separate workflow
• Container CI now triggers on push to master

[codecov]

Codecov Report

Merging #40 (fd5fece) into master (320d1f0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
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  Coverage   96.69%   96.69%           
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  Files          46       46           
  Lines        1424     1424           
=======================================
  Hits         1377     1377           
  Misses         47       47           

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[garryod]

See #40 (review)

[garryod]

1. How do we control when the container workflow is run?
2. Should we add a badge for container build since it's now a separate workflow?

[collang]

The container workflow will trigger on a push to master, which I think covers:

• Changes pushed directly to master
• PR merged with master
  I suppose we could add a badge to show that the containers are working? I don't know whether other repos are set up this way.