Enable local testability

[apparle]

This PR enables the ability to build and test images locally. It does 2 things:

1. Updates the Dockerfile for mastercontainer to separate out the cloning step into its own stage, such that it can be overridden by user input to pick changes from local path with below command. This follows the principle described in https://www.docker.com/blog/dockerfiles-now-support-multiple-build-contexts/

cd Containers/mastercontainer
docker buildx build --build-context aio_root=../../ -t nextcloud/all-in-one:latest .

2. Since the hash of a locally built image will never match the latest image, mandating "Update mastercontainer" makes local testing impossible. This PR adds ability to skip the mastercontainer update. This also addresses allow to restart containers without updating mastercontainer, if a mastercontainer update is available #5468

[szaimen]

Hi @apparle thanks a lot for so many useful contributions! 😊

I just invited you to the repo to make collaboration easier :)

[szaimen]

btw this PR has some conflicts...

[apparle]

btw this PR has some conflicts...

Merged main and resolved conflicts. Also resolved lint issues.

[docjyJ]

I think it would be better to use an environment variable like SKIP_DOMAIN_VALIDATION=true

[apparle]

I think it would be better to use an environment variable like SKIP_DOMAIN_VALIDATION=true

But then it doesn't solve #5468 easily. It would be better to be able to skip the update temporarily rather than take everything down and restart with different env variables. (And then take it down again to remove the env variables when the reason-to-skip is resolved.)