Fix up the container build

[ianmcorvidae]

...to use a multi-stage build, cache better, and most importantly actually keep the library in the final image (fixes #931)

The issue with not keeping the library around was almost certainly my own fault when amending the prior PR for some other changes. My bad there!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.19%. Comparing base (8f0faf5) to head (dd1df47).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #932   +/-   ##
=======================================
  Coverage   63.19%   63.19%           
=======================================
  Files          25       25           
  Lines        4521     4521           
=======================================
  Hits         2857     2857           
  Misses       1664     1664           

Flag	Coverage Δ
unittests	63.19% <ø> (ø)

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