Allow for replacing FROM image in locked down environments

[viper-cool79]

See #333 for background

The purpose is to do a simple replacement of the word placeholder in the Dockerfile. I have read-through the documentation for the github actions.yaml but it seems that there is no easy way to add a --build-args.

The logic is then to run a preparatory shell script to alter the Dockerfile (can most likely be done in a more fashionable manner) and this is here done via sed.

[CLAassistant]

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