Allow defining of Dockerfile by name from working directory

I've been testing out the Bamboo Specs API and Docker tasks.

I'd like to have multiple Dockerfiles in the repository for multiple types of tests. Let's say Dockerfile.unit and Dockerfile.integration, each with different dependencies.

Currently, you can define the Dockerfile in the task either with .dockerfileFromPath() or .dockerfileInWorkingDir().

The latter doesn't have a way of defining the Dockerfile's name. I have a workaround, which uses a relative path "../../Dockerfile.unit", but this reads the Dockerfile and uses it inline during publishing of the spec and not when the task itself is running.