ci: adapt to the new release process

[luislhl]

Acceptance Criteria

• Disable caching of Docker layers when building nightly builds
• Write the BUILD_VERSION file with the version that was just built. This should be done only for nigthly, release-candidate and release builds.
• For the release-candidate, we should only build a Docker image for the default Python version (currently 3.9), and it should have only the main tag on it, not the tags for specific python versions/implementations)
• We should send messages to Slack for builds of release-candidate and release images, but only for the default python version in our version matrix. Otherwise we would be sending many alerts at once.
• Fix a previous error in the workflow where we were apparently making comparisons like if: steps.prep.outputs.login-dockerhub, but we were assign the value 'false' to these variables, which is a truthy value.
• We should have a guide about how to use https://github.com/nektos/act to test our Github Actions workflow

Notes

I'll be proposing a refactoring on this in a new PR (#548) to extract these Python script into actual Python files, because they are becoming too hard to maintain.

Then we will just call the scripts from the workflow and we will be able to have unit tests for them.

TODO

• Create the Slack secrets in the repo

[codecov]

Codecov Report

Merging #547 (25997fe) into dev (2859617) will decrease coverage by 0.06%.
The diff coverage is n/a.

❗ Current head 25997fe differs from pull request most recent head 79a4717. Consider uploading reports for the commit 79a4717 to get more accurate results

@@            Coverage Diff             @@
##              dev     #547      +/-   ##
==========================================
- Coverage   83.31%   83.26%   -0.06%     
==========================================
  Files         212      212              
  Lines       18729    18723       -6     
  Branches     2596     2595       -1     
==========================================
- Hits        15605    15590      -15     
- Misses       2560     2566       +6     
- Partials      564      567       +3     

see 4 files with indirect coverage changes

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