build: Build and publish Docker images for relay and discovery server (fixes #8691, fixes #8960)

This adds builds for the discovery and relay servers in the same manner
as Syncthing, so that Docker images are kept up to date with releases.

Inspired by and closes #8834.

Co-authored-by: Migelo <miha@filetki.si>

Dockerfile.strelaysrv

@@ -1,15 +1,20 @@
 ARG GOVERSION=latest
 FROM golang:$GOVERSION AS builder
+ARG BUILD_USER
+ARG BUILD_HOST
+ARG TARGETARCH
 
 WORKDIR /src
 COPY . .
 
 ENV CGO_ENABLED=0
-ENV BUILD_HOST=syncthing.net
-ENV BUILD_USER=docker
-RUN rm -f strelaysrv && go run build.go -no-upgrade build strelaysrv
+RUN if [ ! -f strelaysrv-linux-$TARGETARCH ] ; then \
+    go run build.go -no-upgrade build strelaysrv ; \
+    mv strelaysrv strelaysrv-linux-$TARGETARCH ; \
+  fi
 
 FROM alpine
+ARG TARGETARCH
 
 EXPOSE 22067 22070
 
@@ -17,7 +22,7 @@ VOLUME ["/var/strelaysrv"]
 
 RUN apk add --no-cache ca-certificates su-exec
 
-COPY --from=builder /src/strelaysrv /bin/strelaysrv
+COPY --from=builder /src/strelaysrv-linux-$TARGETARCH /bin/strelaysrv
 COPY --from=builder /src/script/docker-entrypoint.sh /bin/entrypoint.sh
 
 ENV PUID=1000 PGID=1000 HOME=/var/strelaysrv