cmd/browser: pull the latest docker image on upspinserver restart

This makes sure users of the upspinserver Docker image run the latest
version each time it is restarted.

Change-Id: I817f1b5c9a9cd3944cfcd05b5491040d772ece50
Reviewed-on: https://upspin-review.googlesource.com/14382
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/cmd/browser/gcp.go b/cmd/browser/gcp.go
index 3a0bbe9..71cd070 100644
--- a/cmd/browser/gcp.go
+++ b/cmd/browser/gcp.go
@@ -580,6 +580,7 @@
     [Service]
     Environment="HOME=/home/upspin"
     ExecStartPre=/usr/bin/docker-credential-gcr configure-docker
+    ExecStartPre=/usr/bin/docker pull gcr.io/upspin-containers/upspinserver:latest
     ExecStart=/usr/bin/docker run --rm -u=2000 --volume=/home/upspin:/upspin -p=443:8443 --name=upspinserver gcr.io/upspin-containers/upspinserver:latest
     ExecStop=/usr/bin/docker stop upspinserver
     ExecStopPost=/usr/bin/docker rm upspinserver