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

5.6/Dockerfile.debian

@@ -53,7 +53,7 @@ RUN set -ex; \
 # gpg: key 5072E1F5: public key "MySQL Release Engineering <[email protected]>" imported
 	key='A4A9406876FCBD3C456770C88C718D3B5072E1F5'; \
 	export GNUPGHOME="$(mktemp -d)"; \
-	gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys "$key"; \
+	gpg --batch --keyserver keyserver.ubuntu.com --recv-keys "$key"; \
 	gpg --batch --export "$key" > /etc/apt/trusted.gpg.d/mysql.gpg; \
 	gpgconf --kill all; \
 	rm -rf "$GNUPGHOME"; \