Fix builds with python3 as default python (#490)

* adding py_runtime target to use with envs that have python pointing to py3

python/BUILD

@@ -14,3 +14,16 @@
 package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])  # Apache 2.0
+
+# Python2 is a dependency of rules_docker due to its dependency on
+# containerregistry. To run builds on environments in which 'which python'
+# points to python3, you can use the py_runtime target below.
+# To use this target, simply add the following flag to your build commands:
+# --python_top=//python:py_runtime_2
+# Note you need to have a symlink to a valid python2 version in
+# '/usr/bin/python2' for this to work.
+py_runtime(
+    name = "py_runtime_2",
+    files = [],
+    interpreter_path = "/usr/bin/python2",
+)