Use experimental keyserver hkps://keys.openpgp.org

The SKS keyserver network is vulnerable to spam attacks, and these
attacks started to happen. Downloading a spammed key will break GnuPG
installation "in hard to debug ways". To mitigate this problem, switch
to using a new experimental keyserver that is not part of the SKS
network. This server has its own limitations, but it seems to be the way
to go in the future.

For more information about the SKS network attack:
https://gist.github.com/rjhansen/67ab921ffb4084c865b3618d6955275f

Fixes #72.

docker/develdlang

@@ -15,7 +15,7 @@ apt_add_bintray_repos sociomantic-tsunami/ebtree sociomantic-tsunami/dlang \
 		dlang-community/apt
 
 # Add extra DMD D-APT repo
-apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EBCF975E5BA24D5E
+apt-key adv --keyserver hkps://keys.openpgp.org --recv-keys EBCF975E5BA24D5E
 # Added manually until D-APT is fixed
 #wget http://downloads.sourceforge.net/project/d-apt/files/d-apt.list \
 #               -O /etc/apt/sources.list.d/d-apt.list

docker/util.sh

@@ -70,20 +70,24 @@ apt_update_and_install_base_packages()
 	# Select extra packages depending on the distro version
 	case "$dist" in
 	bionic)
-		extra_packages="gpg-agent dirmngr"
+		extra_packages="gpg-agent"
+		;;
+	xenial)
+		extra_packages="gnupg-agent gnupg-curl"
 		;;
 	*)
 		extra_packages=
 		;;
 	esac
 
 	# We install some basic packages first.
-	apt -y install apt-transport-https software-properties-common curl $extra_packages
+	apt -y install apt-transport-https software-properties-common curl dirmngr \
+			$extra_packages
 }
 
 apt_install_bintray_key()
 {
-	apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 379CE192D401AB61
+	apt-key adv --keyserver hkps://keys.openpgp.org --recv-keys 379CE192D401AB61
 }
 
 apt_add_bintray_repos()