Adds gnupg2 for apt-keys in ubuntu-minimal

Change present in debian-minimal pushed to ubuntu-minimal.

Change-Id: I736bffe6dd9fb57e450505d62c6ca000f678ded2
Closes-Bug: #1889076

diskimage_builder/elements/ubuntu-minimal/root.d/75-ubuntu-minimal-baseinstall

@@ -57,6 +57,8 @@ fi
 
 # default required
 to_install+="busybox sudo " # dib-lint: safe_sudo
+# required for dpkg element to be able to add apt keys
+to_install+="gnupg2 "
 
 if [ "$DIB_PYTHON_VERSION" == "2" ]; then
     to_install+="python "