Run script allows empty docker_args (#2053)

Currently when using `bazel run` on a docker image, I get:
```
/[redacted].executable: line 281: docker_args[@]: unbound variable
```

Alternatively, we could just `set +u` before execing docker.

Author: illicitonion

container/incremental_load.sh.tpl

@@ -283,6 +283,17 @@ if [[ "%{run}" == "True" ]]; then
   # We can clean up before we exec docker, since the exit handler will no longer run.
   cleanup
 
+  # Bash treats empty arrays as unset variables for the purposes of `set -u`, so we only
+  # conditionally add these arrays to our args.
+  args=(%{run_statement})
+  if [[ ${#docker_args[@]} -gt 0 ]]; then
+    args+=("${docker_args[@]}")
+  fi
+  args+=("%{run_tag}")
+  if [[ ${#container_args[@]} -gt 0 ]]; then
+    args+=("${container_args[@]}")
+  fi
+
   # This generated and injected by docker_*.
-  eval exec %{run_statement} "${docker_args[@]}" "%{run_tag}" "${container_args[@]:-}"
+  eval exec "${args[@]}"
 fi