diff --git a/app/build_containers.py b/app/build_containers.py
index 52ba310..7ecf4a8 100644
--- a/app/build_containers.py
+++ b/app/build_containers.py
@@ -103,7 +103,7 @@ def command(ctx, include, exclude):
             # Check if we have a repo for this container. If not, set the context dir to the container-build subdir
             repo_full_path = os.path.join(dev_root_path, repo_dir)
             repo_dir_or_build_dir = repo_dir if os.path.exists(repo_full_path) else build_dir
-            build_command = os.path.join(container_build_dir, "default-build.sh") + f" {container} {repo_dir_or_build_dir}"
+            build_command = os.path.join(container_build_dir, "default-build.sh") + f" {container}:local {repo_dir_or_build_dir}"
         if not dry_run:
             if verbose:
                 print(f"Executing: {build_command}")
diff --git a/app/build_npms.py b/app/build_npms.py
index 101fde3..4503de6 100644
--- a/app/build_npms.py
+++ b/app/build_npms.py
@@ -27,6 +27,8 @@ from python_on_whales import docker, DockerException
 from .base import get_stack
 from .util import include_exclude_check, get_parsed_stack_config
 
+builder_js_image_name = "cerc/builder-js:local"
+
 @click.command()
 @click.option('--include', help="only build these packages")
 @click.option('--exclude', help="don\'t build these packages")
@@ -42,6 +44,8 @@ def command(ctx, include, exclude):
     stack = ctx.obj.stack
     continue_on_error = ctx.obj.continue_on_error
 
+    _ensure_prerequisites()
+
     # build-npms depends on having access to a writable package registry
     # so we check here that it is available
     package_registry_stack = get_stack(ctx.obj, "package-registry")
@@ -95,7 +99,7 @@ def command(ctx, include, exclude):
                 print(f"Executing: {build_command}")
             envs = {"CERC_NPM_AUTH_TOKEN": npm_registry_url_token} | ({"CERC_SCRIPT_DEBUG": "true"} if debug else {})
             try:
-                docker.run("cerc/builder-js",
+                docker.run(builder_js_image_name,
                            remove=True,
                            interactive=True,
                            tty=True,
@@ -126,3 +130,13 @@ def command(ctx, include, exclude):
         else:
             if verbose:
                 print(f"Excluding: {package}")
+
+
+def _ensure_prerequisites():
+    # Check that the builder-js container is available and
+    # Tell the user how to build it if not
+    images = docker.image.list(builder_js_image_name)
+    if len(images) == 0:
+        print(f"FATAL: builder image: {builder_js_image_name} is required but was not found")
+        print("Please run this command to create it: laconic-so --stack build-support build-containers")
+        sys.exit(1)
diff --git a/app/data/container-build/default-build.sh b/app/data/container-build/default-build.sh
index b369e2c..3d5d991 100755
--- a/app/data/container-build/default-build.sh
+++ b/app/data/container-build/default-build.sh
@@ -1,6 +1,9 @@
 #!/usr/bin/env bash
 # Usage: default-build.sh <image-tag> [<repo-relative-path>]
 # if <repo-relative-path> is not supplied, the context is the directory where the Dockerfile lives
+if [[ -n "$CERC_SCRIPT_DEBUG" ]]; then
+    set -x
+fi
 if [[ $# -ne 2 ]]; then
     echo "Illegal number of parameters" >&2
     exit 1