fix: ubuntu16 pre-build container

[gsanchezgavier]

Molecule was installing extra tools over the pre-build container which made the python2 installation failed.
see following verbose log:

TASK [Create Dockerfiles from image names] *************************************
--- before
+++ after: /home/ubuntu/.ansible/tmp/ansible-local-58232knngsotc/tmpx4ij7gqi/Dockerfile.j2
@@ -0,0 +1,11 @@
+# Molecule managed
+
+FROM ghcr.io/newrelic/pkg-installation-testing-action-ubuntu1604:latest
+
+
+RUN if [ $(command -v apt-get) ]; then export DEBIAN_FRONTEND=noninteractive && apt-get update && apt-get install -y python3 sudo bash ca-certificates iproute2 python3-apt aptitude rsync && apt-get clean && rm -rf /var/lib/apt/lists/*; \
+    elif [ $(command -v dnf) ]; then dnf makecache && dnf --assumeyes install /usr/bin/python3 /usr/bin/python3-config /usr/bin/dnf-3 sudo bash iproute rsync && dnf clean all; \
+    elif [ $(command -v yum) ]; then yum makecache fast && yum install -y /usr/bin/python /usr/bin/python2-config sudo yum-plugin-ovl bash iproute rsync && sed -i 's/plugins=0/plugins=1/g' /etc/yum.conf && yum clean all; \
+    elif [ $(command -v zypper) ]; then zypper refresh && zypper install -y python3 sudo bash iproute2 rsync && zypper clean -a; \
+    elif [ $(command -v apk) ]; then apk update && apk add --no-cache python3 sudo bash ca-certificates rsync; \
+    elif [ $(command -v xbps-install) ]; then xbps-install -Syu && xbps-install -y python3 sudo bash ca-certificates iproute2 rsync && xbps-remove -O; fi

This PR adds pre_build_image to that container to prevent this

[rubenruizdegauna]

LGTM. Almost there!!! 💪