Build multiarch images for server

[relnod]

Continuing #785 and #635

The handling of arm/v7 and arm64/v8 is realy weird as xgo and docker buildx use them differently.

I tested it manually in the following way:

$ docker run -w /src -v `pwd`:/src -it --rm --entrypoint "/bin/bash" techknowlogick/xgo
$ apt update
$ apt install tree
$ PLATFORMS="linux|arm64/v8;linux|arm/v7;linux|amd64" make cross-compile-server
$ exit
$ docker buildx build -f docker/Dockerfile.server.multiarch --platform="linux/amd64" -t woodpecker-server .
$ docker buildx build -f docker/Dockerfile.server.multiarch --platform="linux/arm/v7" -t woodpecker-server .
$ docker buildx build -f docker/Dockerfile.server.multiarch --platform="linux/arm64/v8" -t woodpecker-server .

This could be simplified further, but I first want to see if the pipeline works.

[anbraten]

@relnod Looks like a success 😉

[6543]

This could be simplified further, but I first want to see if the pipeline works.

it works ... it stays this way ... we can play around in upcoming pulls 👍

[relnod]

Nice! Thanks for merging so quickly and for the initial work!