Add support for command prefix.

[adamjt]

We use foreman to generate upstart scripts for use on production with Docker. To achieve this we specify a prefix like:

docker run -v /data/tmp:/tmp -e RAILS_ENV=$RAILS_ENV -e PORT=$PORT -e CUSTOM_LOGFILE=STDOUT

That way we can still use foreman for local dev.

[bfulton]

Check out Dockly @ddollar, you can get a sense of how we're using prefixes.

[colszowka]

👍

[ddollar]

In this case I believe you should create a new exporter that supports the docker export rather than adding an additional option to all exports.

[bfulton]

@ddollar, thanks for the feedback and we're happy to open a docker+upstart pull introducing a specialized exporter. The reason this seemed cross-cutting to us is that people are free to use whatever OS and init regime they want inside a container, so this seemed like the best way to leverage existing foreman support for export targets.

Let us know if this changes your opinion. Again, we're happy to take a different approach, just want to figure out the best way to avoid duplication.