upgrade Yarn to v4.0, Node to v20 and fix the docker image test

[igoychev]

Which area(s) of Podkrepi.bg are affected? (leave empty if unsure)

No response

Describe the feature you'd like to request

Two things to upgrade and one to fix:

• Yarn 4.0 is out and will be great to upgrade
• The frontend is updated to Node v20, so let's update in Docker image to v20 too
• The github action that tests the docker images is building the master instead of the PR code, so it needs to be fixed to use the context from the git checkout in the previous step(see screenshot below)

Describe the solution you'd like

The fix of the Docker checkout action is to add "context: ." as per the path context docs after this line here:

api/.github/workflows/security-checks.yml

Line 21 in 6d1bd9f

 with: 

Describe alternatives you've considered

No response