Make Docker container port configurable

[pcrock-thmdo]

Some cloud providers like Heroku assign random ports to Docker containers on startup via a PORT environment variable.

This change allows users (and cloud providers) to more easily configure which port Structurizr runs on without...

• copy / paste / modifying the original CMD in the Dockerfile
• needing to control the --publish Docker CLI parameter

To test, we can use docker run with an --env parameter that sets the port to something other than 8080.

docker build --tag localhost/structurizr-lite .
docker run --rm -it \
    --env PORT=8081 \
    --publish "8081:8081" \
    localhost/structurizr-lite:latest

# in another terminal:
curl --verbose http://localhost:8081/

This should output:

*   Trying 127.0.0.1:8081...
* Connected to localhost (127.0.0.1) port 8081 (#0)
> GET / HTTP/1.1
> Host: localhost:8081
> User-Agent: curl/7.81.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 302 
< Referrer-Policy: strict-origin-when-cross-origin
< X-Frame-Options: deny
< Location: http://localhost:8081/workspace/diagrams
< Content-Language: en-US
< Content-Length: 0
< Date: Tue, 15 Aug 2023 10:32:48 GMT
< 
* Connection #0 to host localhost left intact

[simonbrowndotje]

Thanks!