Fix permission issue encountered on Docker hub

The first builds on docker hub all failed. The error we were getting was

```
[91m ERR! Error: EACCES: permission denied, open '/home/node/package.json'
npm ERR! [Error: EACCES: permission denied, open '/home/node/package.json'] {
npm ERR!�[0m
[91m errno: -13,
npm ERR! code: 'EACCES',
npm �[0m
[91mERR! syscall: 'open',
npm ERR! path: '/home/node/package.json'

...

�[91m The operation was rejected by your operating system.
npm ERR! It is likely you do not have the permissions to access this file as the current user
npm ERR!�[0m
�[91m
npm ERR! If you believe this might be a permissions issue, please double-check the
npm ERR! permissions of the file and its containing directories, or try running
npm ERR! the command again as root/Administrator.
�[0m
�[91m
�[0m
�[91mnpm ERR! A complete log of this run can be found in:
npm ERR!�[0m
�[91m /home/node/.npm/_logs/2021-02-18T09_51_29_644Z-debug.log
�[0m
Removing intermediate container 04acee56b734
The command '/bin/sh -c npm install' returned a non-zero code: 243
```

Some searching of the net indicated that the problem is because the `node` user that is provided in the base image and which we set as the main user does not own the `package.json` files we copy into the image.

It's best practise not to leave the user as `root` in a Docker image, nor to run `npm install` as `root`.

We now understand the reason the `node` user is not the owner of the files is because all `COPY` commands in a Dockerfile are carried out as the `root` user. The suggested fix is to include [chown](https://en.wikipedia.org/wiki/Chown) within your copy statement to ensure once the files are available the non-root user is made the owner.

- nodejs/docker-node#1262 (comment)
- https://stackoverflow.com/a/44766666/6117745

Dockerfile

@@ -49,7 +49,9 @@ EXPOSE $PORT 9229 9230
 
 # Install dependencies first, in a different location for easier app bind mounting for local development. To do this we
 # first copy just the package*.json files from the host
-COPY package.json package-lock.json* ./
+# Note. COPY is always run as the root user in Docker. So, to avoid permission issues we immediately make the node user
+# owner of the copied files
+COPY --chown=node:node package.json package-lock.json* ./
 
 RUN npm install
 
@@ -99,7 +101,9 @@ EXPOSE $PORT
 
 # Install dependencies first, in a different location for easier app bind mounting for local development. To do this we
 # first copy just the package*.json files from the host
-COPY package.json package-lock.json* ./
+# Note. COPY is always run as the root user in Docker. So, to avoid permission issues we immediately make the node user
+# owner of the copied files
+COPY --chown=node:node package.json package-lock.json* ./
 
 RUN npm install