Sync Ruby template image variants to devcontainers/images ruby v4

[Copilot]

The ruby image in devcontainers/images bumped to major version 4 (manifest 4.0.2), leaving every ruby:3-* tag referenced by the Ruby templates unpublished (MISSING). The v4 image also dropped RVM in favor of ruby-build/rubies, so both the templates and their tests required updating.

Changes

Repin image major 3 → 4

• src/ruby/.devcontainer/devcontainer.json
• src/ruby-rails-postgres/.devcontainer/Dockerfile

Re-derive imageVariant.proposals from the image manifest's variants, replacing floating 4-* with concrete 4.0-*; set default to 4.0-trixie. Also cleaned up stray whitespace/formatting.

• src/ruby/devcontainer-template.json
• src/ruby-rails-postgres/devcontainer-template.json

Replace removed RVM permission setup with a rubygems group that grants vscode write access to $GEM_HOME (RVM no longer ships in the v4 image):

# src/ruby-rails-postgres/.devcontainer/Dockerfile
RUN groupadd --system rubygems \
	&& usermod --append --groups rubygems vscode \
	&& chgrp -R rubygems "$GEM_HOME" \
	&& chmod -R g+rwX "$GEM_HOME" \
	&& find "$GEM_HOME" -type d -exec chmod g+s {} +

Update tests to drop RVM/rbenv/nvs assumptions and validate the new ruby-build/rubies layout and gem writability:

• test/ruby-rails-postgres/test.sh — removed the two /usr/local/rvm/gems write-permission checks.
• test/universal/test.sh — swapped rvm/rbenv/nvs checks for ruby-build, added ruby gems are writable, and repointed the version count from /usr/local/rvm/rubies to /usr/local/rubies (expected count 2).

Bump version 6.0.1 → 6.0.2 (patch) in both templates.

// proposals now mirror the manifest variants
"proposals": ["4.0-trixie", "3.4-trixie", "3.3-trixie", "4.0-bookworm", "3.4-bookworm", "3.3-bookworm", "3.4-bullseye", "3.3-bullseye"],
"default": "4.0-trixie"

Validation

npx tsx build/check-image-tags.ts <images> reports zero MISSING tags for both templates; updated test scripts pass against the v4 image; edited JSON formatted with Prettier.

• Fixes Sync template image variants with devcontainers/images #439