diff --git a/app/build_containers.py b/app/build_containers.py
index 7ecf4a8..04cecd6 100644
--- a/app/build_containers.py
+++ b/app/build_containers.py
@@ -83,7 +83,9 @@ def command(ctx, include, exclude):
     container_build_env = {
         "CERC_NPM_URL": "http://gitea.local:3000/api/packages/cerc-io/npm/",
         "CERC_NPM_AUTH_TOKEN": config("CERC_NPM_AUTH_TOKEN", default="<token-not-supplied>"),
-        "CERC_REPO_BASE_DIR": dev_root_path
+        "CERC_REPO_BASE_DIR": dev_root_path,
+        "CERC_HOST_UID": f"{os.getuid()}",
+        "CERC_HOST_GID": f"{os.getgid()}"
     }
 
     def process_container(container):
@@ -106,7 +108,7 @@ def command(ctx, include, exclude):
             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}")
+                print(f"Executing: {build_command} with environment: {container_build_env}")
             build_result = subprocess.run(build_command, shell=True, env=container_build_env)
             if verbose:
                 print(f"Return code is: {build_result.returncode}")
diff --git a/app/build_npms.py b/app/build_npms.py
index 83f87ed..877916b 100644
--- a/app/build_npms.py
+++ b/app/build_npms.py
@@ -116,7 +116,11 @@ def command(ctx, include, exclude):
         if not dry_run:
             if verbose:
                 print(f"Executing: {build_command}")
-            envs = {"CERC_NPM_AUTH_TOKEN": npm_registry_url_token} | ({"CERC_SCRIPT_DEBUG": "true"} if debug else {})
+            # Originally we used the PEP 584 merge operator:
+            # envs = {"CERC_NPM_AUTH_TOKEN": npm_registry_url_token} | ({"CERC_SCRIPT_DEBUG": "true"} if debug else {})
+            # but that isn't available in Python 3.8 (default in Ubuntu 20) so for now we use dict.update:
+            envs = {"CERC_NPM_AUTH_TOKEN": npm_registry_url_token}
+            envs.update({"CERC_SCRIPT_DEBUG": "true"} if debug else {})
             try:
                 docker.run(builder_js_image_name,
                            remove=True,
diff --git a/app/data/container-build/cerc-builder-js/Dockerfile b/app/data/container-build/cerc-builder-js/Dockerfile
index 9f46037..35c9a97 100644
--- a/app/data/container-build/cerc-builder-js/Dockerfile
+++ b/app/data/container-build/cerc-builder-js/Dockerfile
@@ -1,14 +1,30 @@
 # Originally from: https://github.com/devcontainers/images/blob/main/src/javascript-node/.devcontainer/Dockerfile
+# Which depends on: https://github.com/nodejs/docker-node/blob/main/Dockerfile-debian.template
 # [Choice] Node.js version (use -bullseye variants on local arm64/Apple Silicon): 18, 16, 14, 18-bullseye, 16-bullseye, 14-bullseye, 18-buster, 16-buster, 14-buster
 ARG VARIANT=16-bullseye
 FROM node:${VARIANT}
 
+# Set these args to change the uid/gid for the base container's "node" user to match that of the host user (so bind mounts work as expected).
+ARG CERC_HOST_UID=1000
+ARG CERC_HOST_GID=1000
+# Make these values available at runtime to allow a consistency check.
+ENV HOST_UID=${CERC_HOST_UID}
+ENV HOST_GID=${CERC_HOST_GID}
+
 ARG USERNAME=node
 ARG NPM_GLOBAL=/usr/local/share/npm-global
 
 # Add NPM global to PATH.
 ENV PATH=${NPM_GLOBAL}/bin:${PATH}
 
+RUN \
+    if [ ${CERC_HOST_GID} -ne 1000 ] ; then \
+        groupmod -g ${CERC_HOST_GID} ${USERNAME} ; \
+    fi \
+    && if [ ${CERC_HOST_UID} -ne 1000 ] ; then \
+        usermod -u ${CERC_HOST_UID} -g ${CERC_HOST_GID} ${USERNAME} && chown ${CERC_HOST_UID}:${CERC_HOST_GID} /home/${USERNAME} ; \
+    fi
+
 RUN \
     # Configure global npm install location, use group to adapt to UID/GID changes
     if ! cat /etc/group | grep -e "^npm:" > /dev/null 2>&1; then groupadd -r npm; fi \
@@ -39,6 +55,7 @@ RUN mkdir /scripts
 COPY build-npm-package.sh /scripts
 COPY yarn-local-registry-fixup.sh /scripts
 COPY build-npm-package-local-dependencies.sh /scripts
+COPY check-uid.sh /scripts
 ENV PATH="${PATH}:/scripts"
 
 COPY entrypoint.sh .
diff --git a/app/data/container-build/cerc-builder-js/check-uid.sh b/app/data/container-build/cerc-builder-js/check-uid.sh
new file mode 100755
index 0000000..a8cbc32
--- /dev/null
+++ b/app/data/container-build/cerc-builder-js/check-uid.sh
@@ -0,0 +1,21 @@
+#!/bin/bash
+# Make the container usable for uid/gid != 1000
+if [[ -n "$CERC_SCRIPT_DEBUG" ]]; then
+    set -x
+fi
+current_uid=$(id -u)
+current_gid=$(id -g)
+# Don't check if running as root
+if [[ ${current_uid} == 0 ]]; then
+    exit 0
+fi
+# Check the current uid/gid vs the uid/gid used to build the container.
+# We do this because both bind mounts and npm tooling require the uid/gid to match.
+if [[ ${current_gid} != ${HOST_GID} ]]; then
+    echo "Warning: running with gid: ${current_gid} which is not the gid for which this container was built (${HOST_GID})"
+    exit 0
+fi
+if [[ ${current_uid} != ${HOST_UID} ]]; then
+    echo "Warning: running with gid: ${current_uid} which is not the uid for which this container was built (${HOST_UID})"
+    exit 0
+fi
diff --git a/app/data/container-build/cerc-builder-js/entrypoint.sh b/app/data/container-build/cerc-builder-js/entrypoint.sh
index 311cb8c..ab80737 100755
--- a/app/data/container-build/cerc-builder-js/entrypoint.sh
+++ b/app/data/container-build/cerc-builder-js/entrypoint.sh
@@ -1,2 +1,3 @@
 #!/bin/sh
+/scripts/check-uid.sh
 exec "$@"
diff --git a/app/data/container-build/default-build.sh b/app/data/container-build/default-build.sh
index 3d5d991..c52dd04 100755
--- a/app/data/container-build/default-build.sh
+++ b/app/data/container-build/default-build.sh
@@ -11,4 +11,4 @@ fi
 image_tag=$1
 build_dir=$2
 echo "Building ${image_tag} in ${build_dir}"
-docker build -t ${image_tag} ${build_dir}
+docker build -t ${image_tag} --build-arg CERC_HOST_UID=${CERC_HOST_UID} --build-arg CERC_HOST_GID=${CERC_HOST_GID} ${build_dir}