Update target_repo_dir docstring
See
@@ -354,7 +354,8 @@ class Repo2Docker(Application):
target_repo_dir = Unicode(
"",
help="""
- Path inside the image where contents of the repositories are copied to.
+ Path inside the image where contents of the repositories are copied to,
+ and where all the build operations (such as postBuild) happen.
Defaults to ${HOME} if not set
""",
