Optimize CircleCI build

[mtlynch]

This makes several changes to our CircleCI build to speed up the build:

• Uses CircleCI ARM Docker executor so we don't have to emulate through QEMU to build ARMv7 binaries.
• Enables Docker Layer Caching so we can skip rebuilding parts of the Docker build image that haven't changed since last build.
• Defers Docker build args so that they're declared close to when they're first used.
  • I don't know if this is strictly required, but it makes it a little easier to read the code.

Now, it runs in seconds as opposed to previously when each run would take \~30m.

[mtlynch]

Automated comment from CodeApprove ➜

⏳ @jdeanwallace please review this Pull Request