executors: Publish VM image to docker hub

Created by: eseliger

This PR addresses feedback on the binary install mode: It's very hard to acquire the executor VM image today, as you need to dig into our install.sh script for the executor cloud images to figure out HOW to build and import it into ignite, get the contents of the dockerfile and ambient files like autologin.conf, and then repeat that process every time they upgrade, and if we change paths adjust all their scripts. So this PR changes this by publishing a new image called sourcegraph/executor-vm (more generic in case we ever switch away from ignite or ubuntu) to docker hub. The executor will also by default use the image on docker hub that was built for the exact executor version. So next time they use a different binary, ignite would just import the image automatically. Of course they can still do preheating like we do in our VMs, but it's not necessary.

Test plan

Rolled the image out on k8s to validate VMs still start properly.