Update Dockerfile to use pip install and venv

[anargyri]

Description

The docker container was not running properly and moreover it was using the script for generating a conda env.
In this PR, I modify the Dockerfile so that

• dependencies are installed in the environment directly with pip
• the docker container supports both conda and venv, which is selected by the user with a build argument
• the README is accordingly updated.

Related Issues

Checklist:

• I have followed the contribution guidelines and code style for this project.
• I have added tests covering my contributions.
• I have updated the documentation accordingly.
• This PR is being made to staging branch and not to main branch.

[miguelgfierro]

lgtm

[anargyri]

lgtm

I tested python 3.7 with conda and the numpy-base install.

[codecov-commenter]

Codecov Report

Merging #1509 (4a9063d) into staging (3e23511) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           staging    #1509      +/-   ##
===========================================
- Coverage    61.96%   61.95%   -0.01%     
===========================================
  Files           84       84              
  Lines         8368     8369       +1     
===========================================
  Hits          5185     5185              
- Misses        3183     3184       +1     

Impacted Files	Coverage Δ
recommenders/models/vae/standard_vae.py	0.00% <0.00%> (ø)

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