feat(deploy): in-container build Dockerfiles — build the image without a host JDK

[devops-thiago]

Problem Statement

Building the container today requires a host toolchain (JDK 21+, Maven, and GraalVM for the native image) before docker build can run. For the self-hosting audience — the project's core audience — that's real onboarding friction: "install Java to build the Docker image" defeats part of the point of shipping a container.

Proposed Solution

Add multi-stage, in-container build Dockerfiles so docker build is the only prerequisite:

• Dockerfile.jvm-build — stage 1 builds with a Maven+JDK image, stage 2 is the existing runtime layout.
• Dockerfile.native-build — stage 1 builds the native image with a Mandrel/GraalVM builder image, stage 2 the existing micro base.
• Layer-cache the Maven repository (--mount=type=cache) so rebuilds are fast.
• Document both in the README quickstart as the zero-toolchain path; keep the current host-build Dockerfiles for CI/dev speed.
• CI builds the in-container variants at least on release tags so they can't rot.

Pairs with #318 (try-before-self-host onboarding): both lower the "first successful deploy" bar.

Alternatives Considered

• Publish pre-built images only — necessary but not sufficient; self-hosters patching or pinning forks still need a no-toolchain build path.
• Devcontainer — helps contributors, not deployers.

Priority

Nice to have