Release: make gpg-loopback executable

The gpg-loopback utility is used to allow the signing
of tags during the automated release process.

The script needs to be executable for allowing the build
to succeed and sign the tags.

Change-Id: Iec487283bfe0ae5fd5fe78b7859e6ed15ea6cb7c
diff --git a/jenkins-docker/agent-release/Dockerfile b/jenkins-docker/agent-release/Dockerfile
index 2e71073..e83cdc7 100644
--- a/jenkins-docker/agent-release/Dockerfile
+++ b/jenkins-docker/agent-release/Dockerfile
@@ -10,7 +10,7 @@
 
 COPY gerrit-release.sh /usr/local/bin
 COPY gerrit-upgrade-test.sh /usr/local/bin
-COPY gpg-loopback /usr/local/bin/gpg-loopback
+COPY --chmod=555 gpg-loopback /usr/local/bin/gpg-loopback
 
 USER jenkins