chore: Add default virtual environment on PATH for Docker image

[matthewfeickert]

Description

Create a default virtual environment and place it on PATH so that it is active and used to install all libraries. Then, just COPY the virtual environment from the builder image instead of all of /usr/local/ reducing the amount of files copied to only the necessary files.

This has the added benefit of symlinking the installed python3 to the virtualenv PATH, so the equivalent of

ln -s $(command -v python3) /usr/local/venv/bin/python

This approach was recommended by Anthony Sottile in his YouTube video "how to get pip for deadsnakes / docker pythons (intermediate) anthony explains 293". Thanks Anthony!

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add a venv virtual environment to PATH
   - Adding venv also symlinks the python doing the installing to the virtual environment's python
      -  e.g. `ls -l $(which python)` shows `/usr/local/venv/bin/python -> /usr/local/bin/python`
   - Thanks to Anthony Sottile for recommending this approach
   - c.f. https://youtu.be/2Hg5-Hrsa6w
   - c.f. https://github.com/pyhf/cuda-images/pull/3
* Update hadolint GitHub Action to hadolint/hadolint-action@v1.5.0

[codecov]

Codecov Report

Merging #1484 (168ec72) into master (6d3ff05) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1484   +/-   ##
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  Coverage   97.65%   97.65%           
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  Files          63       63           
  Lines        4006     4006           
  Branches      565      565           
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  Hits         3912     3912           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.51% <ø> (ø)
unittests	97.42% <ø> (ø)

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[matthewfeickert]

@lukasheinrich @kratsg I used this approach first in pyhf/cuda-images#3 and it works well there. 👍