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Originally the build was based on Batect for containerized builds; however that project is [no longer maintained[(https://github.com/batect/batect) by its author.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

[jbduncan]

There are a few alternatives to Earthly and containerisation for consistent builds that I think should be looked into:

• Nix: the underlying technology of NixOS.
• mise: a "polyglot tool version manager" that downloads tools from multiple sources, preferring the aqua registry first which lets it download tools more securely, apparently.
• cashapp/hermit: another tool version manager, by engineers at CashApp (the same corporate group as Square).

I only have experience with mise.

Nix seems quite promising too, as being the basis of NixOS, it ensures all installs are at fixed versions and are reproducible IIUC. But I also hear that builds in Nix are slow and the Nix language is arcane.