Nextcloud docker healtheck integration

[col-panic]

A docker service should perform some kind of healthcheck on itself and perform a specific action,
like restarting the service or reporting the problem.

Nextcloud in our case is heavily dependent on

• the database being properly available
• the S3 storage being correctly reachable and usable

to assert this, we should extend nextcloud.yml or the depending docker image with respective
tests.

A general topic on database healthcheck can be found here docker-library/docs#2391, I could not yet find any information on s3 resp. checks.

As for S3 - we could both solve it for this project, and open up an issue for the general Nextcloud docker image,
as it might be interesting for other users too. https://github.com/nextcloud/server/wiki/How-to-test-S3-primary-storage

[acomdigital]

@col-panic Please take a look at the following variant of nextcloud.yml extension.
With some time intervals, the healthcheck.php file checks the availability of DB (by getting dummy data from a current connection) and primary S3 storage (by writing a test file to storage, checking availability, and deleting it).