Stricter permissions for the temporary GnuPG home dir.

Silence a warning.

Dockerfile

@@ -8,7 +8,7 @@ RUN apt-get update && \
         gnupg \
     && \
     export GNUPGHOME=/tmp/gnupghome && \
-    mkdir "$GNUPGHOME" && \
+    mkdir --mode=0700 "$GNUPGHOME" && \
     curl https://packages.cloud.google.com/apt/doc/apt-key.gpg > \
     /usr/share/keyrings/google-cloud.gpg && \
     curl https://www.mongodb.org/static/pgp/server-4.4.asc | \