feat: add devcontainer setup for Flipt v2 development

[markphelps]

Summary

Adds a complete devcontainer configuration for Flipt v2 development, enabling one-click setup in VS Code, GitHub Codespaces, or any devcontainer-compatible environment.

• Dockerfile: Ubuntu 24.04 base with build tools, mise, and pre-created volume mount points with correct ownership
• devcontainer.json: Configures workspace, port forwarding (HTTP 8080, gRPC 9000, Vite 5173), persistent volume caches for Go and mise, and automated tool installation via postCreateCommand
• CI workflow: Builds and validates the devcontainer on any .devcontainer/ changes to prevent regressions

Notable fixes

• Pre-create volume mount points as vscode user so Docker initializes named volumes with correct ownership
• chown safety net in postCreateCommand for pre-existing volumes
• Disable Node.js GPG verification (gpg/keyboxd unavailable in containers)
• Remove node_modules volume mount (root ownership causes permission errors, minimal caching benefit since npm ci does clean installs)
• Fix npx --no → npx --yes for non-interactive package-changed installs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.40%. Comparing base (af4f40e) to head (f01bc18).
⚠️ Report is 1 commits behind head on v2.

Additional details and impacted files

@@           Coverage Diff           @@
##               v2    #5542   +/-   ##
=======================================
  Coverage   60.40%   60.40%           
=======================================
  Files         141      141           
  Lines       13995    13995           
=======================================
  Hits         8453     8453           
  Misses       4821     4821           
  Partials      721      721           

Flag	Coverage Δ
integrationtests	34.32% <ø> (ø)
unittests	51.53% <ø> (ø)

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[erka]

I maybe wrong here but we probably don't need Dockerfile. Mise could be added to devcontainer.json

 "features": {
    "ghcr.io/devcontainers-extra/features/mise:1": {},
    "ghcr.io/devcontainers/features/git:1.3.5": {}
  },