Previously, if the agent had a docker image used by `docker build`, but
the base image is for a different platform, this would lead to the
situation, where the target image uses arm64, but the cockroach binaries
inside are amd64.

This patch adds a few guards to prevent the issue:

* Remove the base image
* Use `--pull` to always pull the latest image
* Explicitly specify the target platform

Release note: None