DOC: Add VERSION argument to docker build instructions

Author: effigies

docs/contributors.rst

@@ -122,7 +122,12 @@ If it is necessary to rebuild the Docker image, a local image named
 ``fmriprep`` may be built from within the working fmriprep
 repository, located in ``~/projects/fmriprep``: ::
 
-    ~/projects/fmriprep$ docker build -t fmriprep .
+    ~/projects/fmriprep$ VERSION=$( python get_version.py )
+    ~/projects/fmriprep$ docker build -t fmriprep --build-arg VERSION=$VERSION .
+
+The ``VERSION`` build argument is necessary to ensure that help text
+can be reliably generated. The ``get_version.py`` tool constructs the
+version string from the current repository state.
 
 To work in this image, replace ``poldracklab/fmriprep:latest`` with
 ``fmriprep`` in any of the above commands.
@@ -232,4 +237,4 @@ An example of how this works is shown here: ::
     (transformation matrices, and six corresponding rotation and translation
     parameters) are estimated before any spatiotemporal filtering using
     `mcflirt` [FSL {fsl_ver}, @mcflirt].
-    """.format(fsl_ver=fsl.Info().version() or '<ver>')
+    """.format(fsl_ver=fsl.Info().version() or '<ver>')