Replace DockerContainerInitializer with a static constructor function, whose methods/configs are populated via a builder

[mieubrisse]

The DockerContainerInitializer is getting unwieldy, and it's yet one more interface the user has to implement. I'd like services created via a builder - something like:

ServiceBuilder.new(dockerImage, testVolumeMountpoint, serviceWrappingFunc, startCmdGeneratingFunc)
   .withUsedPorts(usedPortsMap)
   .withGeneratedFiles(mapOfGeneratedFileKeyToInitializationFunc)
   .withFilesArtifacts(mapOfFilesArtifactsToMountpoints)
   .build()

This way, it's very clear what things are required (e.g. testVolumeMountpoint, dockerImage, etc.) and what things are optional, and the average user doesn't need to fuss about the optional stuff.

[mieubrisse]

Dupe of #8