fix: add missing build steps and update Node in Dockerfile

- Update base image from node:18-slim to node:22-slim (matching
  package.json engines requirement)
- Add git to apt-get install (needed by npm dependencies)
- Add webpack-ext step (builds external bundles)
- Add mkdir -p src/pack (prevents race condition)
- Add pack-prod step (builds application bundles)
- Reorder: build steps before gs-app-server install

Tested on ARM64 (Raspberry Pi 4, Docker 29.4.0) and x86_64 (Ubuntu
24.04, Docker 28.0.4). Without these changes, the container starts
but serves an incomplete app that never finishes loading.

src/dock/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:18-slim
+FROM node:22-slim
 WORKDIR /grid
 COPY package.json /grid
 COPY app.js /grid
@@ -6,9 +6,12 @@ COPY apps /grid/apps
 COPY src /grid/src
 COPY web /grid/web
 COPY bin /grid/bin
-RUN apt-get -y update; apt-get -y install curl
+RUN apt-get -y update; apt-get -y install curl git
 EXPOSE 8080
 RUN npm i
+RUN npm run webpack-ext
+RUN mkdir -p src/pack
+RUN npm run pack-prod
 RUN npm install -g @gridspace/app-server
 RUN rm -rf /grid/bin
 CMD gs-app-server