Replace ADD with COPY instruction in Dockerfile

[PeterDaveHello]

Reference:

• https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#add-or-copy

For other items (files, directories) that do not require ADD’s tar auto-extraction capability, you should always use COPY.

[codecov[bot]]

Codecov Report

Merging #331 (ebb0884) into master (2e5b46b) will increase coverage by 0.32%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #331      +/-   ##
==========================================
+ Coverage   68.56%   68.88%   +0.32%     
==========================================
  Files           8        8              
  Lines         474      495      +21     
==========================================
+ Hits          325      341      +16     
- Misses        149      154       +5     

Impacted Files	Coverage Δ
safety/cli.py	50.00% <0.00%> (-6.85%)	:arrow_down:
safety/errors.py	100.00% <0.00%> (ø)
safety/constants.py	100.00% <0.00%> (ø)
safety/safety.py	92.96% <0.00%> (+6.78%)	:arrow_up:

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[rafaelpivato]

Good point about ADD vs COPY. Nevertheless, I am not sure we need this. It would impact cached layers without actually fixing any bug. The best moment to make this change would be when we happen to change the layers for this image. You can consider adding this as part of #328

[PeterDaveHello]

The cache impact will be only one time, as I'm still working on #328 and they are independent, maybe we can get this merged first?