docs: reduce image size in sample Dockerfile

[wodCZ]

Running chown after COPYing the files in the runner phase creates a new image layer, that has basically same size as the .next directory.

We can achieve the same result using the --chown argument of COPY instruction. This time we don't create a duplicate layer.

Additionally, we can save some space (approx. 1.5 MB) on the telemetry disable row by using the opt out env variable.

I believe this change will be most notable for sites generating a lot of pages statically (in my case, I have around 19k pages and this change saved me ~430 MB).

[leerob]

Thank you! Could you also update the corresponding with-docker example?

[wodCZ]

Ah, great catch, didn't realise it'd be in example too. Thanks!

I've updated the example to match updated documentation ✅

Edit: I've also updated the start script in the docker example, since the container was crashing if the PORT env variable wasn't specified (which the README didn't mention).