Dockerfile as CI environment for pipe steps

[achow101]

Pipe currently only supports the image keyword for the steps. It'd be nice to be able to use the dockerfile keyword as can be done in tasks in order to use a dockerfile for the environment rather than a preexisting image.

[fkorotkov]

Nice idea! But this might take a while to implement since each step can have dockerfile instead of image and then task dependency becomes a bit complex.

We are currently migrating of the internal parser written in Kotlin to a config parser in Go which is open sourced here. Once we'll fully migrate to the new parser (probably by the end of this month), we'll work on this feature so we only need to implement it in Go.

[achow101]

I think a possible workaround would be to have a docker_builder task which builds an image that can be referred to with the image keyword. But I haven't figured out how to do that yet, and it may require #761