cmd/syncthing: Add environment variables for --home, --conf, and --data (fixes #8957) (#8952)

This allows environment overrides for our directories. This is
advantageous because, apart from the obvious, it means we can set it in
the Docker file and not add command line options there. Having the
command line option as we did meant that it was impossible to use the
Docker image for other commands than `serve` (because that is implied
when we see other options on the command line).

Dockerfile

@@ -44,5 +44,6 @@ HEALTHCHECK --interval=1m --timeout=10s \
   CMD curl -fkLsS -m 2 127.0.0.1:8384/rest/noauth/health | grep -o --color=never OK || exit 1
 
 ENV STGUIADDRESS=0.0.0.0:8384
+ENV STHOMEDIR=/var/syncthing/config
 RUN chmod 755 /bin/entrypoint.sh
-ENTRYPOINT ["/bin/entrypoint.sh", "/bin/syncthing", "-home", "/var/syncthing/config"]
+ENTRYPOINT ["/bin/entrypoint.sh", "/bin/syncthing"]