Docker : add a "wait for the db" feature

[romu70]

Hi all.
Currently the Shiori container doesn't start if the DB is not reachable. It would be cool to add a feature which simply waits for the DB availability on a regular basis. This way, we wouldn't have to deal with containers start sequence.

[fmartingr]

I will make a mental note for this, would be interesting to have for a liveness endpoint also. In the meanwhile, you can use a wait-for-it[^1} approach in docker, either you a new container wait for the DB, and the application container wait for this container, or a custom container with this command before the serve command. I believe there's a wait-for-db somewhere too.

[fmartingr]

I've made a proper issue for this: #588

[romu70]

Thx a lot