Error on invalid containers

[seddonym]

Addresses #68.

[codecov-io]

Codecov Report

Merging #73 into master will increase coverage by 1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #73   +/-   ##
=======================================
+ Coverage   89.08%   90.08%   +1%     
=======================================
  Files          11       11           
  Lines         559      565    +6     
=======================================
+ Hits          498      509   +11     
+ Misses         61       56    -5

Impacted Files	Coverage Δ
src/layer_linter/cmdline.py	61.68% <100%> (+4.67%)	⬆️
src/layer_linter/contract.py	92.3% <100%> (+0.37%)	⬆️

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