Add Gerrit OAUTH plugin and setup for GitHub

Using v3.5.1 from
https://github.com/davido/gerrit-oauth-provider/releases/download/v3.5.1/gerrit-oauth-provider.jar
diff --git a/gerrit/Dockerfile.entrypoint-unprivileged b/gerrit/Dockerfile.entrypoint-unprivileged
index 78ca1f8..86574b1 100644
--- a/gerrit/Dockerfile.entrypoint-unprivileged
+++ b/gerrit/Dockerfile.entrypoint-unprivileged
@@ -2,4 +2,13 @@
 
 set -e
 
+if [ "${GITHUB_OAUTH_CLIENT_ID}" -a "${GITHUB_OAUTH_CLIENT_SECRET}" ]; then
+    git config -f /var/gerrit/etc/gerrit.config \
+        plugin.gerrit-oauth-provider-github-oauth.root-url "https://github.com/"
+    git config -f /var/gerrit/etc/gerrit.config \
+        plugin.gerrit-oauth-provider-github-oauth.client-id "${GITHUB_OAUTH_CLIENT_ID}"
+    git config -f /var/gerrit/etc/gerrit.config \
+        plugin.gerrit-oauth-provider-github-oauth.client-secret "${GITHUB_OAUTH_CLIENT_SECRET}"
+fi
+
 exec /entrypoint.sh "$@"