feat: add devcontainer feature and template for Haskell toolchain

[develop7]

Closes #123.

Summary

Adds an official devcontainer feature (haskell-toolchain) and template (haskell) to the repo, replacing the archived microsoft/vscode-dev-containers Haskell artifact.

Feature: haskell-toolchain v1.0.0

Installs the Haskell toolchain via ghcup on any Debian/Ubuntu base image:

Tool	Version
GHC	9.14.1 (LTS)
cabal-install	3.16.1.0
Stack	3.11.1
HLS	2.14.0.0
ghcup	0.2.6.2
ormolu	0.8.2.0
fourmolu	0.20.0.0

8 options: version, installStack, installHLS, installStackGHCupHook, adjustBash, installLinters, globalPackages, globalLibraries.

Design choices

• Version pinning via Nickel: versions.ncl imports from 9.14/bookworm.ncl and 9.14/_globals.ncl — the same data files that generate the Docker images. Single source of truth.
• Non-root install: mirrors devcontainers-extra/features/src/haskell — sudo -iu $_REMOTE_USER with GHCUP_USE_XDG_DIRS=1; root fallback when _REMOTE_USER is unset.
• install.sh is generated from install.sh.tpl.ncl via Nickel (same pattern as the Dockerfiles).
• Multistage Dockerfile.test for cached local testing with podman.

Why only ormolu and fourmolu?

hlint, stylish-haskell, and cabal-fmt constrain ghc < 9.13 or base < 4.22, making them incompatible with GHC 9.14.1 (base 4.22.0.0). They depend on ghc-lib-parser 9.12.x, which has not been updated for GHC 9.14. Users can install them via globalPackages on older GHC versions. See the feature README for details.

No Alpine in v1.0.0

Alpine support is tracked by #22 and will land in lockstep with the Alpine base images.

Template: haskell

Clone-and-go cabal helloworld project:

• Base: mcr.microsoft.com/devcontainers/base:debian-12
• VS Code extensions: haskell.haskell, haskell-lang-server
• Format-on-save with fourmolu
• postCreateCommand: cabal update && cabal build

CI

Workflow	Trigger	Purpose
feature-test.yml	PR + push	4-job matrix (debian/ubuntu × amd64/arm64) + shellcheck
feature-publish.yml	tag feature-v*	OCI publish to ghcr.io/haskell/docker-haskell/devcontainer-features/haskell-toolchain
template-lint.yml	PR	Validate template metadata
feature-versions-sync.yml	weekly cron	Check for version drift, open issue on staleness

Build layers are cached via GitHub Actions cache (type=gha), so changes to test.sh or the workflow do not rebuild the toolchain.

How to verify (as a devcontainers user)

Option A: Use the template

# Clone this PR branch
git clone --branch feat/devcontainer https://github.com/develop7/docker-haskell.git
cd docker-haskell/devcontainer-template/src/haskell

# Open in VS Code with the Dev Containers extension
# (or use `devcontainer up --workspace-folder .`)

The container builds, installs the toolchain via ghcup, runs cabal update && cabal build, and you should have a working Haskell environment with GHC 9.14.1, HLS, ormolu, and fourmolu.

Option B: Use the feature in your own project

Create .devcontainer/devcontainer.json in any repo:

{
  "image": "mcr.microsoft.com/devcontainers/base:debian-12",
  "features": {
    "ghcr.io/haskell/docker-haskell/devcontainer-features/haskell-toolchain:1.0.0": {}
  }
}

Then devcontainer up --workspace-folder . or "Reopen in Container" in VS Code. Verify:

ghc --version          # → 9.14.1
cabal --version        # → 3.16.1.0
stack --version        # → 3.11.1
haskell-language-server-wrapper --version  # → 2.14.0.0
ormolu --version       # → 0.8.2.0
fourmolu --version     # → 0.20.0.0

Option C: Local Docker/podman test

git clone --branch feat/devcontainer https://github.com/develop7/docker-haskell.git
cd docker-haskell

# Build and test in one shot
podman build -t haskell-feature-test \
  -f devcontainer-feature/src/haskell-toolchain/Dockerfile.test \
  devcontainer-feature

# Or with a different base image
podman build --build-arg BASE_IMAGE=ubuntu:24.04 -t haskell-feature-ubuntu \
  -f devcontainer-feature/src/haskell-toolchain/Dockerfile.test \
  devcontainer-feature

Test plan (internal)

• nickel export renders install.sh cleanly (no errors, correct substitutions)
• shellcheck -x passes on all shell scripts (zero warnings)
• devcontainer features test passes (_default scenario on mcr.microsoft.com/devcontainers/base:debian-12)
• podman build with multistage Dockerfile.test — all tools verified:
  • ghc --version → 9.14.1
  • cabal --version → 3.16.1.0
  • stack --version → 3.11.1
  • haskell-language-server-wrapper --version → 2.14.0.0
  • ormolu --version → 0.8.2.0 (ghc-lib-parser 9.14.1)
  • fourmolu --version → 0.20.0.0 (ghc-lib-parser 9.14.1)
• Root path (no _REMOTE_USER) — tools install under /root, test.sh detects and passes
• Non-root path (_REMOTE_USER=vscode) — tools install under /home/vscode, test.sh detects and passes
• CI: debian:bookworm × {amd64,arm64} + ubuntu:24.04 × {amd64,arm64} all pass
• CI: shellcheck job passes
• CI: lint (template metadata validation) passes
• CI: generate (Dockerfile generator) passes
• Reviewer: verify versions.ncl import paths resolve from devcontainer-feature/src/haskell-toolchain/ to 9.14/bookworm.ncl
• Reviewer: verify devcontainer-template/src/haskell/helloworld.cabal base ^>=4.22 matches GHC 9.14.1
• Reviewer: spot-check install.sh.tpl.ncl heredoc variable expansion (outer shell expands pinned versions, \$-escapes protect inner shell vars)