Support externally maintained DNF cache

Signed-off-by: Chris Evich <chris_gitlab@icuc.me>

Containerfile

@@ -37,16 +37,20 @@ ARG EXCLUDE_PACKAGES="\
 # Base-image runs as user 'podman', temporarily switch to root
 # for installation/setup.
 USER root
-# Not a real build-arg.  Avoiding addition of an env. layer
-# only to help prevent some extra typing.
-ARG dnfcmd="dnf --setopt=tsflags=nodocs -y"
+# Helper for comparison in future RUN operations (DO NOT USE)
+ARG _DNFCMD="dnf --setopt=tsflags=nodocs -y"
+# Set this instead, if (for example) you want to volume-mount in /var/cache/dnf
+ARG DNFCMD="${_DNFCMD}"
+# Avoid installing any documentation to keep image small
 # During install, excluding packages is meaningless if already installed
 RUN set -x && \
     rm -f /etc/dnf/protected.d/sudo.conf && \
     rm -f /etc/dnf/protected.d/yum.conf && \
-    $dnfcmd remove ${EXCLUDE_PACKAGES} && \
-    dnf clean all && \
-    rm -rf /var/cache/dnf
+    $DNFCMD remove ${EXCLUDE_PACKAGES} && \
+    if [[ "${DNFCMD}" == "${_DNFCMD}" ]]; then
+      dnf clean all && \
+      rm -rf /var/cache/dnf; \
+    fi
 
 # Enable callers to customize the runner version as needed, otherwise
 # assume this image will be version-tagged, so it's fine to grab the latest.
@@ -56,11 +60,13 @@ ARG TARGETARCH="amd64"
 ENV RUNNER_RPM_URL=https://gitlab-runner-downloads.s3.amazonaws.com/${RUNNER_VERSION}/rpm/gitlab-runner_${TARGETARCH}.rpm
 RUN for rpm in ${EXCLUDE_PACKAGES}; do x+="--exclude=$rpm "; done && \
     set -x && \
-    $dnfcmd update && \
-    $dnfcmd install $x $RUNNER_RPM_URL && \
-    $dnfcmd upgrade && \
-    dnf clean all && \
-    rm -rf /var/cache/dnf
+    $DNFCMD update && \
+    $DNFCMD install $x $RUNNER_RPM_URL && \
+    $DNFCMD upgrade && \
+    if [[ "${DNFCMD}" == "${_DNFCMD}" ]]; then
+      dnf clean all && \
+      rm -rf /var/cache/dnf; \
+    fi
 
 # In case of a runner escape, prevent easy installation of packages.
 RUN rm -f /etc/dnf/protected.d/* && \