feat: Add postgresql database support

[claneys]

Hello,

I find your repo quite useful and just wanted to use postgresql db instead. So I did some changes to make it works. I also added some improvements. I use it in kubernetes at the moment and works nice.

It makes image heavier though, maybe could be better to split in a separate images.

[dorianim]

Thanks for the Update @claneys ! I've looked at some other containers from linuxserver.io, and noticed, that they do not prefix the database variables there. So I'd prefer not to do that either. Is there any good reason to do so?

[claneys]

I still have some problems but it should improve from your side. I have to deal with some issue from my environment before being certain that it fixes the problems.

[dorianim]

Unfortunately, it still doesn't work properly. It only works after removing the getenv('MRBS_DB_*') ?? part. I think, it would be easier to just not add the MRBS prefix and leave it as it is.

[claneys]

Ok found. Have to dig a bit on how php retrieve its env var, explanation in the commit message :

Null coalescence operator ?? works on 'null' value, undefined variable, and getenv() returns false or something so that variable is defined with an empty but not undefined value.
So using a ternary operator works better on using getenv() function. We could use $_ENV but env var could be found in $_ENV or $_SERVER depending which is the
source of the environment variable, it seems more reliable to use getenv() then.

This is a bit more verbose way to write it but works every times.

[dorianim]

I decided to drop the MRBS prefix. Thanks for your contribution!