Fail-fast the release process if the PGP key is unable to sign

The execution of the Gerrit release process is conditioned on the
success of the PGP signature. Fail immediately upon release if the key
and passphrase do not allow to perform a digital signature, so that the
process fails immediately.

Change-Id: Id23cf54b1e87a1078a4e02718151202f04e4a3d0

Author: lucamilanesio

jenkins-docker/agent-release/gerrit-release.sh

@@ -84,6 +84,11 @@ then
   export GPG_PASSPHRASE_FILE="$HOME/.gnupg/gpg-passphrase"
   echo '$GPG_PASSPHRASE' | envsubst '$GPG_PASSPHRASE' > $GPG_PASSPHRASE_FILE
   git config --global gpg.program /usr/local/bin/gpg-loopback
+
+
+  echo "Testing if GPG signature works"
+  echo foo > /tmp/foo
+  /usr/local/bin/gpg-loopback --sign /tmp/foo && /usr/local/bin/gpg-loopback --verify /tmp/foo.gpg
 fi
 
 GPG_USER=$(gpg -K --with-colons | grep uid | cut -d ':' -f 10)