refactor(dev-tools): always update /devtools

[sjinks]

1. Copy dev-tools to both /dev-tools-orig and /dev-tools (the latter destination is required for backward compatibility).
2. When the container starts, sync tools from /dev-tools-orig to /dev-tools.

This will simplify the logic in VIP CLI (it will not have to find and remove devtools volume every time the environment starts) and ensure that the volume has the latest tools.

VIP CLI PR: Automattic/vip-cli#2139

Update:
3. Switch the base image from alpine to busybox and add a static build of rsync. This reduces the image size from 11.8MB to 3.05MB and eliminates the need to update the image weekly (busybox updates less often and has fewer security vulnerabilities than a full-fledged distro).
4. Make the build process more cache-friendly by making copied files independent on their own layer.

[github-actions]

Trivy Scan Report

Trivy Returned Empty Report

[sjinks]

1. The latest commit switches the base image from alpine to busybox and adds a static build of rsync. This reduces the image size from 11.8MB to 3.05MB and eliminates the need to update the image weekly (busybox updates less often and has fewer security vulnerabilities than a full-fledged distro).
2. It also makes the build process more cache-friendly by making copied files independent on their own layer.

These updates are friendly to VIP CLI and don't require any changes to the configuration.

[sjinks]

Merging, because Automattic/vip-cli#2139 is already merged.