resolve unrecognized x.y r versions to x.y.0

R.deb download step requires a full x.y.z version, it will 404 on `r-4.4`

Author: minrk

repo2docker/buildpacks/r.py

@@ -99,6 +99,9 @@ def r_version(self):
                 # available. Users can however explicitly specify the full version to get something specific
                 if r_version in version_map:
                     r_version = version_map[r_version]
+                elif len(r_version.split(".")) == 1:
+                    # must have x.y.z version, add .0 for unrecognized (future) R versions
+                    r_version += ".0"
 
             # translate to the full version string
             self._r_version = r_version