DEV: Remove use of `--squash` flag and switch to buildx (#743)

DEV: Remove use of `--squash` flag and switch to buildx (#743)

Why this change?

In CI, we are seeing the following warning message:

```
WARNING: experimental flag squash is removed with BuildKit. You should squash inside build using a multi-stage Dockerfile for efficiency.
```

Basically, the `--squash` flag has not been working for quite some time
and is redundant.

What does this change do?

* This change removes the use of the `--squash` flag.

* This change uses the `buildx` tool in `auto_build.rb` as we prepare to
  build images for multiple platforms.