[Spike] Alternatives to Earthly for containerized builds

[binkley]

Context

Originally the build was based on Batect for containerized builds; however that project is [no longer maintained[(https://github.com/batect/batect) by its author.

This project has moved to Earthly that is well-maintained. However, it currently has some features that may interact poorly depending on project setup and configuration.

An obvious approach is a plain Docker container (something that both Batect and Earthly both use under the hood). Since we rely on Gradle and Maven, and plugins are the key for build quality, the 3rd-party wrappers may not offer enough value to be the default.

See earthly/earthly#4220 for an Earthly issue that came up after following our advice in the pages.

Tasks

• Research alternatives to Earthly, and decide which should be documented
• Provide a plain Dockerfile for a containerized build easily used with local and CI builds
• Decide on: plain Docker files, or use the Earthly wrapper as the project default
• If changing to plain Docker, update doc pages to reflect that, and in one page talk about alternatives

Acceptance criteria

• I always have a green build in CI regardless of container choices
• Reading the docs, I can choose and switch my container while keeping a green CI build