Fix Docker images CI workflows

[jpraynaud]

Sometimes the Docker images creation does not work in the CI:

• The branch name is not correctly extracted thus the image name is not prefixed
• The docker build is not working in that case as the image name is not valid
• Example of failed job (https://github.com/input-output-hk/mithril/runs/6030302226?check_suite_focus=true#step:7:104)
• Example of suceeded job (https://github.com/input-output-hk/mithril/runs/6020800464?check_suite_focus=true#step:6:29)

To do for all docker image jobs:

• Use the latest version of the github actions (the one currently used targets specific versions)
• Remove the env vars in the job as it does look to be mandatory
• Is the problem in the Extract branch name step. Maybe this needs to be refactored

[jpraynaud]

It seems like the problem is only on PR runs (where the branch name is not easily accessible) and it is still here.

We should use something like this for the expression that computes the branch name:
${{ github.head_ref || github.ref_name }}

See these resources:

• https://docs.github.com/en/actions/learn-github-actions/contexts#github-context
• https://stackoverflow.com/questions/58033366/how-to-get-the-current-branch-within-github-actions