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

3.8-rc/alpine/Dockerfile

@@ -10,7 +10,7 @@ RUN apk add --no-cache \
 		procps
 
 # Default to a PGP keyserver that pgp-happy-eyeballs recognizes, but allow for substitutions locally
-ARG PGP_KEYSERVER=ha.pool.sks-keyservers.net
+ARG PGP_KEYSERVER=keyserver.ubuntu.com
 # If you are building this image locally and are getting `gpg: keyserver receive failed: No data` errors,
 # run the build with a different PGP_KEYSERVER, e.g. docker build --tag rabbitmq:3.8 --build-arg PGP_KEYSERVER=pgpkeys.eu 3.8/ubuntu
 # For context, see https://github.com/docker-library/official-images/issues/4252

3.8-rc/ubuntu/Dockerfile

@@ -13,7 +13,7 @@ RUN set -eux; \
 	gosu nobody true
 
 # Default to a PGP keyserver that pgp-happy-eyeballs recognizes, but allow for substitutions locally
-ARG PGP_KEYSERVER=ha.pool.sks-keyservers.net
+ARG PGP_KEYSERVER=keyserver.ubuntu.com
 # If you are building this image locally and are getting `gpg: keyserver receive failed: No data` errors,
 # run the build with a different PGP_KEYSERVER, e.g. docker build --tag rabbitmq:3.8 --build-arg PGP_KEYSERVER=pgpkeys.eu 3.8/ubuntu
 # For context, see https://github.com/docker-library/official-images/issues/4252