documentation enhancement: capability SYS_NICE

[borg1622]

In the Nextcloud documentation: Installation and server configuration > Server tuning > Previews is the following statement:

Make sure to start Imaginary with the -return-size command line parameter. Otherwise, there will be a minor performance impact. The flag requires a recent version of Imaginary (newer than v1.2.4) and is by default added to the aio-imaginary container. Also make sure to add the capability SYS_NICE via –cap-add=sys_nice or cap_add: - SYS_NICE as it is required by imaginary to generate HEIC previews.

As I don't use nextcloud-aio I've installed Imaginary without using docker.
In this context I stumbled over the third sentence which advises to
add the capability SYS_NICE via –cap-add=sys_nice.
It took me some research that this statement is docker related and therefore not relevant in my case.

I therefore suggest that this statement be clarified so that its ties to Docker is made clear.
i.e.

Also make sure to add the docker container capability SYS_NICE via –cap-add=sys_nice or cap_add: - SYS_NICE as it is required by imaginary to generate HEIC previews.

P.S.: This was likely introduced by the following pull request #9724

[skjnldsv]

Valid. In admin_manual/installation/server_tuning.rst (line 255-256), the note reads:

Also, ensure to add the capability SYS_NICE via --cap-add=sys_nice or (for Compose) cap_add: - SYS_NICE, as it is required by Imaginary to generate HEIC previews.

The flags --cap-add and cap_add are Docker/container-specific, but the sentence doesn't say that. Someone running Imaginary as a plain binary (no container) would be confused about what this means for them.

Fix: prefix with something like "When running in a Docker container," so it's clear this only applies to containerized deployments.