dockerfile: Improve UID/GID handling (fixes #5180) (#5181)
dockerfile: Improve UID/GID handling (fixes #5180) (#5181)

This removes the user and group juggling, which would fail when given
for example a PGID that already existed as the "syncthing" group could
then not be created with that PGID. It's not reasonable to expect the
user to know which group/user names/IDs are already present in the
Docker image.

Instead we now just launch under the specified IDs, while manually
setting the HOME env var to give us a home directory - the only thing we
needed the user entry for anyway.

Also updates to Go 1.11 and building without upgrades instead of
disabling by env var.
imsodin/syncthing,wweich/syncthing,syncthing/syncthing,xduugu/syncthing,uok/syncthing,AudriusButkevicius/syncthing,xduugu/syncthing,rumpelsepp/syncthing,uok/syncthing,wweich/syncthing,rumpelsepp/syncthing,xduugu/syncthing,imsodin/syncthing,rumpelsepp/syncthing,imsodin/syncthing,uok/syncthing,imsodin/syncthing,syncthing/syncthing,AudriusButkevicius/syncthing,imsodin/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,syncthing/syncthing,rumpelsepp/syncthing,wweich/syncthing,syncthing/syncthing,uok/syncthing,wweich/syncthing,xduugu/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing