Generalise docker image creation for single instance db across versions (from 18c)

The current workflow to create a single instance db docker image needs to have one dockerfile for each version which is stored with its set of files under that specific folder. We propose to have just one dockerfile that can be provided with a goldimage name as an argument to create the docker image for the base release of that particular goldimage.

[gvenzl]

Assigning this ticket to the development team.
There are good reasons to keep Dockerfiles for each version given the supported lifetimes.
Whether or not this should be unified has implications that the dev team has to deal with and be OK with.