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/pre-delete

@@ -18,7 +18,7 @@ apps_pre_delete() {
   fi
 
   if [[ -d $CACHE_DIR ]]; then
-    docker run "$DOKKU_GLOBAL_RUN_ARGS" --rm -v "$CACHE_HOST_DIR:/cache" "$IMAGE" find /cache -depth -mindepth 1 -maxdepth 1 -exec rm -Rf {} \; || true
+    "$DOCKER_BIN" run "$DOKKU_GLOBAL_RUN_ARGS" --rm -v "$CACHE_HOST_DIR:/cache" "$IMAGE" find /cache -depth -mindepth 1 -maxdepth 1 -exec rm -Rf {} \; || true
   fi
 }