docker: set platform for build target

[mkuf]

This fixes the failed docker Builds caused by #949.

As a bonus, this also speeds up the build process itself.
The nature of the node application allows the generated static files to be used on multiple architectures. Therefore, they only need to be built once and then copied into the platform specific runtime images.

Have a look at the test build I did:

• Pipeline: https://github.com/mkuf/mainsail/runs/7250954266?check_suite_focus=true
• Image: https://github.com/mkuf/mainsail/pkgs/container/mainsail/27950035?tag=edge

-Markus

[pataar]

Woah, this is way better than my solution in #950

[pataar]

Thanks for your contribution Markus, nice solution! I really like https://github.com/mkuf/prind :)

[mkuf]

Thanks, I'm happy you like it :)