Include PORT env variable in "dokku run"

Application deploy includes: the following argument:

    -e PORT="$INT_PORT"

However when "dokku run" is executed, it does not set the same variable (I'm using buildstep).

As a result, if I attempt to initialise my environment similarly to a deploy, it errors out. If it could only set the port then "run" command would make a wonderful debug tool for the boot toolchain. This PR adds the missing variable.