Use simple USER directive.
Using `addgroup` in final `arm` image when building on amd64 doesn't work.
I must have made a mistake during cross build verification.
Alternative is to use qemu-static but it's not worth it for this.
Run as non-root user and group
In the unlikely event that you are currently persisting data to disk then this
change may break file read/write access due to a change in the UID/GID that the
oauth2_proxy process runs as.
Run as non-root system user and group `oauth2proxy` with UID/GID `2000` to avoid clashing with typical local users.
An alternative to creating a separate user is to ~~chown binary and~~ run as `USER nobody`, which also works, can amend this PR if required.
Least access privileges.
Close: https://github.com/pusher/oauth2_proxy/issues/78
Locally with Docker (`-version`):
```
$ ps aux | grep oauth2
2000 25192 6.0 0.0 0 0 ? Ds 15:53 0:00 [oauth2_proxy]
```
Running in Kubernetes 1.13 with the following also specified:
```
securityContext:
readOnlyRootFilesystem: true
runAsNonRoot: true
runAsUser: 10001
```
```
$ kubectl exec -it -n oauth2-proxy oauth2-proxy-85c9f58ffc-dz9lr sh
/opt $ whoami
whoami: unknown uid 10001
/opt $ ps aux
PID USER TIME COMMAND
1 10001 0:00 /opt/oauth2_proxy --whitelist-domain=.example.com --cookie-domain=example.com --email-domain=example.com --upstream=file:///dev/null --http-address=0.0.0.0:4180
11 10001 0:00 sh
17 10001 0:00 ps aux
```
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