Check for pgres startup completed, by checking a file in a shared volume

[y0urself]

What:

Why:

• In the postgres image startup script we restart postgres, gvmd fails if it connects to postgres before restart.
• In the postgres startup script a file is touched after stopping the postgres process and before the restart
• thus we now wait until the file exists and wait in another loop until the process is started.

How did you test it:

Checklist:

• Tests
• PR merge commit message adjusted