Generic IOC image generation should require main branch of ibek-support

[gilesknap]

This issue is about generic IOCs and therefore could be in https://github.com/epics-containers/ioc-template but I'm puting it here because it's really an issue with management of this repo.

Because ibek-support is a submodule of every generic IOC and submodule versions are defined by commit hash only, there is a very likely scenario that individual branches of the this repo will proliferate on a per-generic-IOC basis. The only motivation to merge into main is when you want to re-use changes to a support module definition in a different generic IOC. At that point you may be in a world of merge conflict pain. giles as already experienced this when he is the only person working on the repo at present.

@GDYendell has suggested the following fix.

1. Change the generic IOC CI such that it will fail and not release push image to the container registry if it determines that the commit hash that it is using is NOT in the main branch. This therefore requires that the developer merges and pushes their changes before pushing the generic IOC source.
2. Allow that this check is switched off if the generic IOC tag that kicked off the CI is a beta tag

@coretl care to comment on this?

[coretl]

Sounds reasonable