Support externally maintained DNF cache

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

README.md

@@ -64,18 +64,19 @@ $ eval $(podman inspect --format=json $IMAGE | jq -r .[].Labels.run)
 
 This image may be built simply with:
 
-`podman build -t runner .`
+`podman build -t registry.gitlab.com/qontainers/pipglr:latest .`
 
 This will utilize the latest stable version of podman and the latest
 stable version of the gitlab runner.
 
-### Multi-arch
+### Notes
 
-Assuming the host supports foreign-architecture emulation.  The
-`Containerfile` may be used to produce a multi-arch manifest-list.
-For example:
+* If you wish to use the `testing` or `upstream` flavors of the podman base image,
+  simply build with `--build-arg FLAVOR=testing` (or `upstream`).
 
-`podman build --jobs 4 --platform linux/s390x,linux/ppc64le,linux/amd64 --manifest runner .`
+* Additionally or alternatively, you may specify a specific podman base image tag
+  with `--build-arg BASE_TAG=<value>`.  Where `<value>` is either `latest`, the
+  podman image version (e.g. `v4`, `v4.2`, `v4.2.0`, etc.)
 
 ### Build-args
 
@@ -90,13 +91,18 @@ Several build arguments are available to control the output image:
   and `vX.Y.Z` (where, `X`, `Y`, and `Z` represent the podman semantic
   version numbers).  It's also possible to specify an image SHA.
 * `EXCLUDE_PACKAGES` - A space-separated list of RPM packages to prevent
-  their existance in the final image.  This is intended as a security measure
+  their existence in the final image.  This is intended as a security measure
   to limit the attack-surface should a gitlab-runner process escape it's
   inner-container.
 * `RUNNER_VERSION` - Allows specifying an exact gitlab runner version.
   By default the `latest` is used, assuming the user is building a tagged
   image anyway.  Valid versions may be found on the [runner
   release page](https://gitlab.com/gitlab-org/gitlab-runner/-/releases).
+* `DNFCMD` - By default this is set to `dnf --setopt=tsflags=nodocs -y`.
+  However, if you'd like to volume-mount in `/var/cache/dnf` then you'll
+  need to use
+  `--build-arg DNFCMD="dnf --setopt=tsflags=nodocs -y --setopt keepcache=true`"
+  Note: Changing `DNFCMD` will cause build-time cache cleanup to be disabled.
 * `TARGETARCH` - Supports inclusion of non-x86_64 gitlab runners.  This
    value is assumed to match the image's architecture.  If using the
    `--platform` build argument, it will be set automatically.