feat: allow setting DOCKER_BIN path for docker execution

This change allows operators to specify a DOCKER_BIN environment variable. This will specify a binary to run when executing docker, which is useful in cases where the 'docker' command being run must be modified in a way that would otherwise be invasive to Dokku, but minimalistic if done within a wrapper.

plugins/apps/post-delete

@@ -16,7 +16,7 @@ app_post_delete() {
   fi
 
   # shellcheck disable=SC2046
-  docker rmi $(docker images -q "$IMAGE_REPO" | xargs) &>/dev/null || true
+  "$DOCKER_BIN" rmi $("$DOCKER_BIN" images -q "$IMAGE_REPO" | xargs) &>/dev/null || true
 }
 
 app_post_delete "$@"