Enable multiplatform build (add arm support)

[siberianlove]

Based on issue #510

Since used base image (ruby) has arm support it's pretty easy to add arm support here too. Setup QEMU and specify supported platforms.

Tested on my m1 machine:

Not only through docker cli, but ui is working fine too.

For debugging i published image on my docker hub account through my fork of this repo. You can check it too if you want to:
https://hub.docker.com/r/siberianlove/redmine/tags

[frank-dspeed]

It is a good thing in general but as far as i know i need to recheck we would maybe need a total other image name for arm images will look into that the change in general is correct. i Only need to verify that with docker hub that efficent tagging would be possible @sameersbn maybe look also into that do we want to publish multi arch?

[jcormier]

@siberianlove, I had looked at this last year and started adding support but at the time the blocker was the base ubuntu image and getting certain packages installed. As you mention with the ruby image this suddenly gets very easy to do.

Your changes look look good to me and the github auto checks had no issues building it which is comforting.

I think the only thing missing is an update to the README and a arm64 specific docker compose file for testing. At the time I had written the following document update though if you could update it and make sure it works that's be nice.

Could you grab and update if needed the following commits:
6bb9ca8
d8829bc -- ignore the mysql8 update, I think that needs to be done separately and with some care.

[jcormier]

@frank-dspeed, from my previous research i do believe this should work as written for docker hub