Set GID and UID via ENV variables

[wollsi]

Add convenient way to set UID and GID of postgres user and group via ENV variables.

In some cases UID/GID 999 is already blocked by another user (for example openmediavault uses 999 as UID for the openmediavault user). This results in postgres docker container using the already existing user.

This Pull request also adresses issue #709

[tianon]

I'm confused - what does this actually fix, or even change? The end result of this will be identical to what we currently support except that the resulting image will have two additional environment variables which are confusing to users because setting them at runtime won't actually do anything. 😕

[wollsi]

I'm confused - what does this actually fix, or even change? The end result of this will be identical to what we currently support except that the resulting image will have two additional environment variables which are confusing to users because setting them at runtime won't actually do anything. 😕

Actually you are right. What i inteded to do will not be solved by this PullRequest, thanks for having a look at it! I tried to add the possibility to set uid and gid like in the images provided by linuxserver.io. In the meantime I got my container running with the help of the --user flag (as mentioned by the portgres docker hub site)

Thanks for the second thought! Will close the PullRequest.