feat: add support for immutable image

[thefirstofthe300]

Pull Request

Description of the change

Nextcloud is capable of being built using an immutable image (see https://github.com/thefirstofthe300/nextcloud-docker). In this scenario, there is no point in mounting the /var/www and /var/www/html directories to store state. The remaining directories should be mounted, but not these two.

Benefits

Allows users to build their own immutable images which do not perform an rsync on start but still use this chart.

Possible drawbacks

The default values are not changing so I see no drawbacks.

Additional information

• [X] DCO has been signed off on the commit.
• [X] Chart version bumped in Chart.yaml according to semver.
• [X] (optional) Variables are documented in the README.md

[jessebot]

Does anything bad happen if those volumes are mounted anyway? It looks like you linked a custom image, which you can absolutely use, but I'm unsure if we support this. Either way, the branch needs to be rebased.

@provokateurin are you aware of an immutable mode for nextcloud? I don't really see any issues with merging this, but I also don't have a quick way to test it.

[provokateurin]

Nextcloud is not really immutable. It will probably run into quite a few issues because all code assumes the whole dir is writable.