init/new: publish prebuilt devcontainer image to ghcr.io to avoid ~5min local build

[kaspergff]

Summary

The devcontainer generated by mxcli init / mxcli new builds its image locally from a Dockerfile (cmd/mxcli/tool_templates.go). The first build is slow — on the order of 5+ minutes — because it pulls and installs a large toolchain:

• ~228 MB of apt packages
• Adoptium Temurin JDK 21 (required by MxBuild)
• Node.js 22
• postgresql-client, kafkacat
• Playwright CLI + bundled Chromium / headless-shell

Every contributor and every fresh clone pays this cost on first open (and again whenever the base layers are invalidated).

Proposal

Publish a prebuilt devcontainer image to ghcr.io (e.g. ghcr.io/mendixlabs/mxcli-devcontainer:<tag>) built in CI, and have the generated devcontainer.json reference it via "image" instead of building the Dockerfile locally.

Benefits:

• Fast container startup — pull a cached image instead of a multi-minute build
• Reproducible, CI-verified toolchain (JDK, Node, Playwright versions pinned once)
• Less that can go wrong on the user's machine during first-run

Considerations:

• Versioning/tagging strategy so a project pins an image matching its mxcli / Mendix version
• Multi-arch (amd64 + arm64) build in CI
• Keep the Dockerfile as the buildable source of truth; the published image is built from it
• Podman-specific setup (rootless podman, vfs storage, etc. — see the podman branch of generateDockerfile) must be preserved in the published image or layered appropriately

This is a larger, independent improvement from the postCreateCommand bug fixes; filing separately as an enhancement.