devcontainer: skip legacy docker-compose in docker-in-docker

[bilby91]

Fix (continuation of #89, #90)

#90 pinned dockerDashComposeVersion: v2, and the v2 plugin (docker compose) did install — but the released docker-in-docker feature also installs the legacy docker-compose (hyphen) command. On bookworm/arm64 that goes through a global pip install docker-compose, which fails under PEP 668:

error: externally-managed-environment
ERROR: Feature "Docker (Docker-in-Docker)" failed to install!

amd64 keeps passing because it gets a prebuilt Compose-v1 binary (no pip).

Why none is safe

• The repo only uses docker compose v2 — runtime/docker/compose.go states "legacy docker-compose is not supported" and execs the v2 plugin.
• The v2 plugin comes from the moby-compose apt package, installed independently of this option (already succeeded in the devcontainer: use Compose v2 in docker-in-docker #90 run).
• none is gated cleanly (if [ "$DOCKER_DASH_COMPOSE_VERSION" != "none" ]) and simply skips the hyphen command + its pip path.

Status of the prebuild chain

• devcontainer: pin prebuild base to bookworm #89 (bookworm): unblocked amd64.
• devcontainer: use Compose v2 in docker-in-docker #90 (compose v2 plugin): installed the plugin, but the hyphen command still pip-failed on arm64.
• This: drops the hyphen command → removes the last arm64 pip failure. amd64 already builds green, so this should let the multi-arch :latest manifest publish.

Only verifiable by the DevContainer Prebuild run on main after merge.

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between e8d9ab7 and 66fcdec.

📒 Files selected for processing (1)

• .devcontainer/devcontainer-build.json

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-dind-compose-none

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