fix(dataflow): add SELDON_POD_NAMESPACE env var for Docker deployments

[adriangonz]

What this PR does / why we need it:
Following #5072, the pod.namespace arg is now mandatory for the Dataflow engine. However, this wasn't reflected on the Docker Compose files for local deployments, causing the `Dataflow container to crash at start time with:

Exception in thread "main" com.natpryce.konfig.Misconfiguration: pod.namespace property not found; searched:
 - --pod-namespace in command-line parameters
 - SELDON_POD_NAMESPACE in environment variables
 - pod.namespace in resource local.properties

        at com.natpryce.konfig.Configuration$get$1.invoke(konfig.kt:91)
        at com.natpryce.konfig.Configuration$get$1.invoke(konfig.kt:85)
        at com.natpryce.konfig.Configuration$DefaultImpls.getOrElse(konfig.kt:109)
        at com.natpryce.konfig.Override.getOrElse(konfig.kt:314)
        at com.natpryce.konfig.Configuration$DefaultImpls.get(konfig.kt:91)
        at com.natpryce.konfig.Override.get(konfig.kt:314)
        at io.seldon.dataflow.Main.main(Main.kt:39)

This PR adds a default SELDON_POD_NAMESPACE env var on the Docker Compose config to make sure the Dataflow engine can come up.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer: