Switch away from deprecated keyserver

As per https://central.sonatype.org/publish/requirements/gpg/#distributing-your-public-key
the sks-keyservers.net is deprecated. I could no longer build the images
using them. I updated them to use keyserver.ubuntu.com

Author: AlistairB

8.10/buster/Dockerfile

@@ -39,8 +39,8 @@ ARG STACK_KEY=C5705533DA4F78D8664B5DC0575159689BEFB442
 ARG STACK_RELEASE_KEY=2C6A674E85EE3FB896AFC9B965101FF31C5C154D
 
 RUN export GNUPGHOME="$(mktemp -d)" && \
-    gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys ${STACK_KEY} && \
-    gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys ${STACK_RELEASE_KEY} && \
+    gpg --batch --keyserver keyserver.ubuntu.com --recv-keys ${STACK_KEY} && \
+    gpg --batch --keyserver keyserver.ubuntu.com --recv-keys ${STACK_RELEASE_KEY} && \
     curl -fSL https://github.com/commercialhaskell/stack/releases/download/v${STACK}/stack-${STACK}-linux-x86_64.tar.gz -o stack.tar.gz && \
     curl -fSL https://github.com/commercialhaskell/stack/releases/download/v${STACK}/stack-${STACK}-linux-x86_64.tar.gz.asc -o stack.tar.gz.asc && \
     gpg --batch --trusted-key 0x575159689BEFB442 --verify stack.tar.gz.asc stack.tar.gz && \

8.10/stretch/Dockerfile

@@ -39,8 +39,8 @@ ARG STACK_KEY=C5705533DA4F78D8664B5DC0575159689BEFB442
 ARG STACK_RELEASE_KEY=2C6A674E85EE3FB896AFC9B965101FF31C5C154D
 
 RUN export GNUPGHOME="$(mktemp -d)" && \
-    gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys ${STACK_KEY} && \
-    gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys ${STACK_RELEASE_KEY} && \
+    gpg --batch --keyserver keyserver.ubuntu.com --recv-keys ${STACK_KEY} && \
+    gpg --batch --keyserver keyserver.ubuntu.com --recv-keys ${STACK_RELEASE_KEY} && \
     curl -fSL https://github.com/commercialhaskell/stack/releases/download/v${STACK}/stack-${STACK}-linux-x86_64.tar.gz -o stack.tar.gz && \
     curl -fSL https://github.com/commercialhaskell/stack/releases/download/v${STACK}/stack-${STACK}-linux-x86_64.tar.gz.asc -o stack.tar.gz.asc && \
     gpg --batch --trusted-key 0x575159689BEFB442 --verify stack.tar.gz.asc stack.tar.gz && \

9.0/buster/Dockerfile

@@ -39,8 +39,8 @@ ARG STACK_KEY=C5705533DA4F78D8664B5DC0575159689BEFB442
 ARG STACK_RELEASE_KEY=2C6A674E85EE3FB896AFC9B965101FF31C5C154D
 
 RUN export GNUPGHOME="$(mktemp -d)" && \
-    gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys ${STACK_KEY} && \
-    gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys ${STACK_RELEASE_KEY} && \
+    gpg --batch --keyserver keyserver.ubuntu.com --recv-keys ${STACK_KEY} && \
+    gpg --batch --keyserver keyserver.ubuntu.com --recv-keys ${STACK_RELEASE_KEY} && \
     curl -fSL https://github.com/commercialhaskell/stack/releases/download/v${STACK}/stack-${STACK}-linux-x86_64.tar.gz -o stack.tar.gz && \
     curl -fSL https://github.com/commercialhaskell/stack/releases/download/v${STACK}/stack-${STACK}-linux-x86_64.tar.gz.asc -o stack.tar.gz.asc && \
     gpg --batch --trusted-key 0x575159689BEFB442 --verify stack.tar.gz.asc stack.tar.gz && \

9.0/stretch/Dockerfile

@@ -39,8 +39,8 @@ ARG STACK_KEY=C5705533DA4F78D8664B5DC0575159689BEFB442
 ARG STACK_RELEASE_KEY=2C6A674E85EE3FB896AFC9B965101FF31C5C154D
 
 RUN export GNUPGHOME="$(mktemp -d)" && \
-    gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys ${STACK_KEY} && \
-    gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys ${STACK_RELEASE_KEY} && \
+    gpg --batch --keyserver keyserver.ubuntu.com --recv-keys ${STACK_KEY} && \
+    gpg --batch --keyserver keyserver.ubuntu.com --recv-keys ${STACK_RELEASE_KEY} && \
     curl -fSL https://github.com/commercialhaskell/stack/releases/download/v${STACK}/stack-${STACK}-linux-x86_64.tar.gz -o stack.tar.gz && \
     curl -fSL https://github.com/commercialhaskell/stack/releases/download/v${STACK}/stack-${STACK}-linux-x86_64.tar.gz.asc -o stack.tar.gz.asc && \
     gpg --batch --trusted-key 0x575159689BEFB442 --verify stack.tar.gz.asc stack.tar.gz && \