Catch applying multiple operations to the same dataset

[RobbeSneyders]

Branching pipelines are not yet supported, but we're not longer properly validating the pipeline for branches. When applying multiple operations to the same dataset with the docker runner, the following error is thrown:

Error response from daemon: driver failed programming external connectivity on endpoint dummy-pipeline-chunk_text-1 (8a33bda10be26230ac89930e4b7af55134776ba3d7470a60078d088d6c762117): Bind for 0.0.0.0:8787 failed: port is already allocated