Debian: T7023: download smoketest container images only once

When setting up vyos-1x-smoketest package, the required container images will
be fetched from the appropriate registry. During development one will re-install
the vyos-1x generated packages periodically. In the past this triggered a
re-download of the container images for every set-up of the package.

  Getting image source signatures
  Copying blob sha256:d3a4026919f923f4e0bb9a23a1e5c2d3c5593d31cbac8d2d6d032285b4852945
  Copying config sha256:c1f39daffdeffeb97987901406e2ecef0fb2c2ca236fdfaf570d088426294d91
  Writing manifest to image destination
  Storing signatures

  Getting image source signatures
  Copying blob sha256:a0d0a0d46f8b52473982a3c466318f479767577551a53ffc9074c9fa7035982e
  Copying blob sha256:064e2154c8ec1ddeb114ebc9db9a3876ee8883e9a14fe8622c31cb6f17b759f6
  Copying blob sha256:7e3fbb46165bc5a98b12c136087a13992e30fe00ab4fab2bbe6c7edd657d8c5b
  Copying blob sha256:80a416511ac029206f3f824a15b1c94845c410242a1e463c466a1b3081f7e20f
  Copying blob sha256:339be6688c410f9851f6f09cf0c9d63819f8ca5f2bb09d93ce8c42714842f5ed
  Copying config sha256:6950ba3bd4492642b6c6c0c5f5bb88a5f2a48f700974a2bdba74333a65d9324e
  Writing manifest to image destination
  Storing signatures

This change will download the container images only if the image is not present
on the system.

debian/vyos-1x-smoketest.postinst

@@ -2,14 +2,12 @@
 
 BUSYBOX_TAG="docker.io/library/busybox:stable"
 BUSYBOX_PATH="/usr/share/vyos/busybox-stable.tar"
-if [[ -f $BUSYBOX_PATH ]]; then
-    rm -f $BUSYBOX_PATH
+if [[ ! -f $BUSYBOX_PATH ]]; then
+    skopeo copy --additional-tag "$BUSYBOX_TAG" "docker://$BUSYBOX_TAG" "docker-archive:/$BUSYBOX_PATH"
 fi
-skopeo copy --additional-tag "$BUSYBOX_TAG" "docker://$BUSYBOX_TAG" "docker-archive:/$BUSYBOX_PATH"
 
 TACPLUS_TAG="docker.io/lfkeitel/tacacs_plus:alpine"
 TACPLUS_PATH="/usr/share/vyos/tacplus-alpine.tar"
-if [[ -f $TACPLUS_PATH ]]; then
-    rm -f $TACPLUS_PATH
+if [[ ! -f $TACPLUS_PATH ]]; then
+    skopeo copy --additional-tag "$TACPLUS_TAG" "docker://$TACPLUS_TAG" "docker-archive:/$TACPLUS_PATH"
 fi
-skopeo copy --additional-tag "$TACPLUS_TAG" "docker://$TACPLUS_TAG" "docker-archive:/$TACPLUS_PATH"