Support R full version provided by user

i.e. MAJOR.MINOR.PATCH.

repo2docker/buildpacks/r.py

@@ -101,7 +101,12 @@ class RBuildPack(PythonBuildPack):
                 if r_version in version_map:
                     r_version = version_map[r_version]
                 else:
-                    # Instead of appending ".0" to future R versions,
+                    r_version_parts = r_version.split(".")
+                    if len(r_version_parts) == 3:
+                        warnings.warn(
+                            f"Using R full version, {r_version}, provided by user."
+                        )
+                    else:
                         # repo2docker fails earlier with a meaningful message to the user.
                         # If repo2docker doesn't fail here, repo2docker might fail later
                         # without a meaningul message to the user.