Switch from SKS to Ubuntu keyserver

See also https://github.com/docker-library/faq#openpgp--gnupg-keys-and-verification and https://github.com/tianon/pgp-happy-eyeballs

Author: tianon

10/Dockerfile

@@ -29,7 +29,7 @@ ENV GPG_KEYS \
 
 RUN set -ex; \
 	for key in $GPG_KEYS; do \
-		gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys "$key"; \
+		gpg --batch --keyserver keyserver.ubuntu.com --recv-keys "$key"; \
 	done
 
 # https://gcc.gnu.org/mirrors.html