emailservice: remove explicit PORT env variable

[ahayworth]

Changes

This is a follow-up to #248, which in turn was resolving: #242 (comment)

The ruby app is relying on convention, and by convention a PORT env
varable will determine which port the puma webserver actually listens
on. So - we don't need to set it explicitly in the compose file, but
that means we need to set it elsewhere somehow.

I tried a few hacks with the dockerfile:

• Trying to pass PORT=$EMAIL_SERVICE_PORT
• Trying to pass -p $EMAIL_SERVICE_PORT in the command

But those didn't work and I gave up. So instead we just set it directly
in the sinatra app ourselves. (Please don't ask me how that actually
gets set down all the way through sinatra -> rack -> puma, because
truthfully I do not know: rack-based servers are basically magic).

We can see that it works:

@ahayworth ➜ /workspaces/opentelemetry-demo-webstore (ahayworth/emailservice-port-things ✗) $ git grep EMAIL_SERVICE_P
ORT
.env:EMAIL_SERVICE_PORT=6060

@ahayworth ➜ /workspaces/opentelemetry-demo-webstore (ahayworth/emailservice-port-things ✗) $ docker-compose up --build emailservice
email-service            | == Sinatra (v2.2.0) has taken the stage on 6060 for production with backup from Puma
email-service            | I, [2022-08-03T13:08:27.643291 #1]  INFO -- : Instrumentation: OpenTelemetry::Instrumentation::Sinatra was successfully installed with the following options {}
email-service            | Puma starting in single mode...
email-service            | * Puma version: 5.6.4 (ruby 3.1.2-p20) ("Birdie's Version")
email-service            | *  Min threads: 0
email-service            | *  Max threads: 5
email-service            | *  Environment: production
email-service            | *          PID: 1
email-service            | * Listening on http://0.0.0.0:6060
email-service            | Use Ctrl-C to stop

cc @mic-max

[mic-max]

Thanks!