Specify need for using py2 with Bazel

[mattjmcnaughton]

The default python on my system is python3. This caused me to run into
bazelbuild/rules_docker#454 when running
bazel build //.... Update the documentation to indicate that
/usr/bin/env python must resolve to python2 in order for all of the
Bazel commands included in the documentation to work.

We can remove this comment after Bazel better supports using python3 as
the default system python. It appears from
bazelbuild/rules_docker#580 that the
rules_docker folks hope to merge the fix in mid to late Q1 2019.

[k8s-ci-robot]

Hi @mattjmcnaughton. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mattjmcnaughton]

/assign @stevekuznetsov

thanks :)

[fejta]

/ok-to-test
/lgtm
/approve
thanks

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fejta, mattjmcnaughton

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