Support for running in Docker

[vnegi10]

• Add a Dockerfile to build an image
• Rework remove_old_files() function to handle dates
• Update test data for CI

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -4.88% ⚠️

Comparison is base (1f9da29) 72.61% compared to head (89dd853) 67.74%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   72.61%   67.74%   -4.88%     
==========================================
  Files           7        7              
  Lines         493      496       +3     
==========================================
- Hits          358      336      -22     
- Misses        135      160      +25     

Files Changed	Coverage Δ
src/helpers.jl	76.85% <100.00%> (-20.30%)	⬇️

... and 1 file with indirect coverage changes

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