From what I have observed, ha.pool.sks-keyservers.net is not very reliable and I have failed to receive keys on several occasions.
I think we should add alternative mirrors, something like:
RUN set -ex; \
for key in $GPG_KEYS; do \
gpg --keyserver pgp.mit.edu --recv-keys "$key" || \
gpg --keyserver keyserver.pgp.com --recv-keys "$key" || \
gpg --keyserver ha.pool.sks-keyservers.net --recv-keys "$key" ; \
done