Installing bazelisk in official bazel docker's image

[Or Shachar]

We really like using the official bazel image "l.gcr.io/google/bazel" in our CI builds.

For local builds we use Bazelisk and `.bazelversion` file.

A really good feature of those images would be to also have bazelisk such that:

- If there's a match between .bazelversion (or USE_BAZEL_VERSION env) and the image tag -it will use the existing binary

- Else - it will download the required binary

This will allow us to easily test newer versions by simply changing the .bazelversion file and opening a pullrequest.

There's also an issue about it in bazelisk https://github.com/bazelbuild/bazelisk/issues/90

WDYT?