Install the python-is-python3 package in our docker build

[droazen]

This symlinks /usr/bin/python to /usr/bin/python3, which can help avoid issues with scripts that reference the python binary directly.

Resolves #8402

[droazen]

@lbergelson and/or @gokalpcelik please review

[gokalpcelik]

Looks like python-is-python3 is not available to ubuntu bionic :-1: my bad on the bad choice of packages.

[droazen]

I didn't realize we were still on Ubuntu 18.04 in the base image -- that is quite old. We should look into updating to at least 20.04, if not 22.04.