chore: Add local-testing Dockerfile and build script for feature branch testing in docker

docker/Dockerfile.local

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+# Local-testing variant of docker/Dockerfile.
+# Instead of installing the published @flowfuse/device-agent from npm, it
+# installs a tarball built from the local working copy via `npm pack`.
+#
+# Build context MUST be the device-agent repo root so the .tgz is reachable:
+#   cd packages/device-agent
+#   npm pack
+#   docker build -f docker/Dockerfile.local -t flowfuse/device-agent:local .
+ARG NODE_VERSION=24
+FROM node:${NODE_VERSION}-alpine
+
+ARG FF_UID=2000
+ARG FF_GID=2000
+
+RUN apk add --no-cache --virtual buildtools build-base linux-headers udev python3 openssl
+
+RUN addgroup -g ${FF_GID} -S flowfuse \
+    && adduser -u ${FF_UID} -S -G flowfuse -h /opt/flowfuse-device flowfuse \
+    && mkdir -p /opt/flowfuse-device \
+    && chown -R "${FF_UID}":"${FF_GID}" /opt/flowfuse-device
+
+# Copy the locally-built package tarball into the image and install it globally.
+COPY flowfuse-device-agent-*.tgz /tmp/device-agent.tgz
+RUN npm install -g npm \
+    && npm config set cache /opt/flowfuse-device/.npm --global \
+    && npm install -g /tmp/device-agent.tgz --omit=dev \
+    && rm -f /tmp/device-agent.tgz \
+    && chown -R ${FF_UID}:${FF_GID} /opt/flowfuse-device
+
+EXPOSE 1880
+
+ENV HOME=/opt/flowfuse-device
+
+USER flowfuse
+
+CMD ["flowfuse-device-agent"]

docker/build-local.md

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+## Local feature branch testing
+
+The image published to a registry installs the released `@flowfuse/device-agent`
+package from npm. When testing **local code changes** (e.g. on a feature branch),
+you can use the provided helper script to build an image from your branch instead.
+
+The script uses [`Dockerfile.local`](./Dockerfile.local), which packages the local
+source with `npm pack` and installs that tarball — mirroring how the production
+image installs the published package, but with your changes.
+
+### Building
+
+From the device-agent repo root, run the script:
+
+```bash
+# Linux / macOS / Windows Subsystem for Linux (WSL)
+# optional: make the script executable:
+sudo chmod +x ./docker/build-local.sh
+# then run:
+./docker/build-local.sh
+```
+
+The script will:
+
+- remove any stale `flowfuse-device-agent-*.tgz`
+- run `npm pack` to package the local working copy
+- build and tag the image based on the current git branch:
+  - on `main` → `flowfuse/device-agent:local-build`
+  - on any other branch &rarr; `flowfuse/device-agent:<branch>-local-build`
+    (branch lowercased, illegal tag characters replaced with `-`)
+
+You can override the defaults:
+
+```bash
+IMAGE_NAME=myorg/device-agent ./docker/build-local.sh   # override repo name
+TAG=custom-tag ./docker/build-local.sh                   # override the whole tag
+```
+
+### Running
+
+Run exactly as the published image, just using the locally-built tag and
+mounting your `device.yml`:
+
+```bash
+docker run --rm -it -v /path/to/device.yml:/opt/flowfuse-device/device.yml -p 1880:1880 flowfuse/device-agent:local-build
+```
+
+Re-running a build script reassigns the tag to the freshly built image; the
+previous build becomes a dangling (`<none>`) image. A running container is not
+updated automatically — stop it and `docker run` again to pick up a rebuild.
+Clean up old dangling images occasionally with `docker image prune -f`.

docker/build-local.sh

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+#!/usr/bin/env bash
+# Builds a local-testing image of the device-agent from the current working copy.
+#
+#   - clears any stale flowfuse-device-agent-*.tgz
+#   - runs `npm pack` to package the local code
+#   - tags the image based on the current git branch:
+#       main            -> flowfuse/device-agent:local-build
+#       <other-branch>  -> flowfuse/device-agent:<branch>-local-build
+#
+# Usage (from anywhere):
+#   ./docker/build-local.sh
+#   IMAGE_NAME=myorg/device-agent ./docker/build-local.sh   # override repo name
+#   TAG=custom-tag ./docker/build-local.sh                   # override the whole tag
+
+# Fail fast: -e exits on any command error, -u errors on unset variables,
+# -o pipefail makes a pipeline fail if any stage (not just the last) fails.
+set -euo pipefail
+
+# Generate a default image name and tag if not supplied by the caller.
+IMAGE_NAME="${IMAGE_NAME:-flowfuse/device-agent}"
+TAG="${TAG:-}"
+
+# Resolve the device-agent repo root (parent of this script's /docker dir) and work from there.
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(dirname "$SCRIPT_DIR")"
+cd "$REPO_ROOT"
+
+# 1. Clear any old tarballs so only the fresh pack remains.
+for f in flowfuse-device-agent-*.tgz; do
+    if [ -e "$f" ]; then
+        echo "Removing old pack: $f"
+        rm -f "$f"
+    fi
+done
+
+# 2. Package the local working copy.
+echo "Running npm pack..."
+npm pack
+
+# 3. Work out the tag from the branch name (unless one was supplied).
+if [ -z "$TAG" ]; then
+    branch="$(git rev-parse --abbrev-ref HEAD)"
+    if [ "$branch" = "main" ]; then
+        TAG="local-build"
+    else
+        # Sanitise: lowercase, and replace anything not allowed in a docker tag with '-'
+        safe_branch="$(echo "$branch" | tr '[:upper:]' '[:lower:]' | sed 's/[^a-z0-9_.-]/-/g')"
+        TAG="${safe_branch}-local-build"
+    fi
+fi
+
+FULL_IMAGE="${IMAGE_NAME}:${TAG}"
+
+# 4. Build the image (context = repo root, so the .tgz is reachable by COPY).
+echo "Building image: $FULL_IMAGE"
+docker build -f docker/Dockerfile.local -t "$FULL_IMAGE" .
+
+echo ""
+echo "Built $FULL_IMAGE"
+echo ""
+echo "Enter the following command to run the device-agent:"
+echo "  docker run --rm -it \\"
+echo "    -v /opt/flowfuse-device-docker-local/device.yml:/opt/flowfuse-device/device.yml \\"
+echo "    -p 1888:1880 \\"
+echo "    $FULL_IMAGE"
+echo ""
+echo "NOTE:"
+echo "Entering the above command will run the device-agent on port 1888"
+echo "using your local device.yml (edit as needed before hitting enter)"