Docker has tag in image name

Docker labels are used for metadata not versions

docs/adding_models.rst

@@ -21,7 +21,7 @@ Adding a new version of a model
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 A model can have different versions.
-A model version in the eWaterCycle Python package corresponds to the label of Docker image and the version in a Singularity container image filename.
+A model version in the eWaterCycle Python package corresponds to the tag of Docker image and the version in a Singularity container image filename.
 The version of the container image should preferably be one of release versions of the model code. Alternativly the version could be the name of a feature branch or a date.
 
 Also parameter sets can be specify which versions of a model they support.