Add gpg to Debian Docker image for our test

We add gpg into our Docker image (and benefit from the cache effect)
instead of running molecule prepare step to install that.

This only concern the Debian image, because it's already available
on RockyLinux base image.

.config/molecule/Dockerfile.j2

@@ -6,7 +6,7 @@ ENV DEBIAN_FRONTEND noninteractive
 
 {% if item.image.startswith('debian') %}
 RUN apt-get update \
-    && apt-get install -y python3 sudo bash ca-certificates iproute2 python3-apt aptitude systemd systemd-sysv \
+    && apt-get install -y python3 sudo bash ca-certificates iproute2 python3-apt aptitude systemd systemd-sysv gpg \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 {% else %}