Use a different PGP keyserver for OpenSSL PGP keys

Certain keyservers fail seasonally & intermittently - every few months
one stops working reliabley - so then we alternate between them. Last
time we had this problem, pgpkeys.eu was failing, and now pgpkeys.uk
seems to be failing and resulting in flaky builds:

https://github.com/rabbitmq/rabbitmq-server/runs/2663558141?check_suite_focus=true

Trying pgpkeys.eu now, and preparing to go to the Ubuntu one next if
this one proves to not be as reliable as we would like it to be.

As a different approach, I am wondering if we should store these PGP
keys somewhere instead? WDYT @dumbbell?

Thanks @pjk25 for flagging this!

Signed-off-by: Gerhard Lazu <[email protected]>

Author: gerhard

packaging/docker-image/Makefile

@@ -35,13 +35,16 @@ endif
 OTP_VERSION ?= 23.2.4
 OTP_SHA256 ?= e72aa084907e0f34f932cf00caa33aba93147b0a7c9c35569d6bd1c402f532de
 REPO ?= pivotalrabbitmq/rabbitmq
+PGP_KEYSERVER ?= pgpkeys.eu
+ALT1_PGP_KEYSERVER ?= keyserver.ubuntu.com
+ALT2_PGP_KEYSERVER ?= pgpkeys.uk
 
 all: dist
 
 dist:
 	xzcat $(GENERIC_UNIX_ARCHIVE) | tar xvf -
 	docker build --pull \
-	  --build-arg PGP_KEYSERVER=pgpkeys.uk \
+	  --build-arg PGP_KEYSERVER=$(PGP_KEYSERVER) \
 	  --build-arg OTP_VERSION=$(OTP_VERSION) \
 	  --build-arg OTP_SHA256=$(OTP_SHA256) \
 	  --build-arg RABBITMQ_BUILD=rabbitmq_server-$(VERSION) \