Getting started with Bazel Docker Container has wrong information about python3 version

### Description of the problem / feature request:

[Bazel container starter guide](https://github.com/bazelbuild/bazel/blob/master/site/docs/bazel-container.md) has wrong information about python3 version

### Feature requests: what underlying problem are you trying to solve with this feature?

reproduce guide and replace output in https://github.com/bazelbuild/bazel/blob/master/site/docs/bazel-container.md with correct. Also, it'd be good to have image with python3.6+ installed.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.
```
MacBook-Pro-Evgenij:~ evgenijdedov$ docker run -it --entrypoint=/bin/bash l.gcr.io/google/bazel:latest
```
```
Unable to find image 'l.gcr.io/google/bazel:latest' locally
latest: Pulling from google/bazel
db4d8d6ad7f4: Pull complete
f9bfe4cb548a: Pull complete
3c2cba919283: Pull complete
fe16a83246ad: Pull complete
443bf173a96b: Pull complete
Digest: sha256:e8b0adc7d9b8cf6ef28ae8d8c575169a9affeaca89fec2fced0e38c8d08e5059
Status: Downloaded newer image for l.gcr.io/google/bazel:latest
```
```
root@d60d37afbbd1:/#
root@d60d37afbbd1:/# python3 -V
Python 3.5.2
```
```
root@d60d37afbbd1:/# bazel version
Extracting Bazel installation...
Build label: 2.0.0
Build target: bazel-out/k8-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar
Build time: Thu Dec 19 12:30:18 2019 (1576758618)
Build timestamp: 1576758618
Build timestamp as int: 1576758618
root@d60d37afbbd1:/#
```
But I expected different python version as promised in starter guide:
```
root@5a99103747c6:/# python3 -V
Python 3.6.6
```

information on [image page](https://console.cloud.google.com/marketplace/details/google/bazel?filter=category:developer-tools&id=e629cad7-9b87-4006-88ed-7725e80b1170&pli=1) at Google Cloud says that image includes python 3.5

Reproduce the command and Python version check described in site/docs/bazel-container.md using the referenced l.gcr.io/google/bazel:latest image. Compare the observed output with the guide, update the documented output if needed, and verify whether the requested Python 3.6+ image change is part of the same work.

Written by the indexing model from the issue text.