MUL-2375 fix(docker): pass VERSION, COMMIT, DATE build args to docker build

[sapk]

What does this PR do?

Forward Makefile build variables (VERSION, COMMIT, DATE) to Docker builds so that binaries inside Docker images report accurate version information instead of hardcoded defaults (dev/unknown).

Currently, docker-compose.selfhost.build.yml builds the backend image without passing any build args, so the resulting multica and server binaries fall back to version=dev and commit=unknown. The Makefile already computes these values from git metadata.

Add ARG DATE=unknown to the Dockerfile and pass -X main.date= in the multica binary ldflags similar to make build.

Related Issue

None

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Refactor / code improvement (no behavior change)
• Documentation update
• Tests (adding or improving test coverage)
• CI / infrastructure

Changes Made

• Dockerfile — Added ARG DATE=unknown and -X main.date=${DATE} to the multica build ldflags (matching the Makefile build target pattern)
• docker-compose.selfhost.build.yml — Added args: block to the backend service build config, forwarding VERSION, COMMIT, and DATE from the environment (which the Makefile export already sets)

How to Test

1. Build via Makefile: make selfhost-build
2. Verify the binary version inside the image:

   docker run --rm --entrypoint=/app/multica multica-backend:dev version

   Should output a real version (e.g. v0.2.20 or v0.0.0-20260516083000-abc123def456), not dev.
3. Verify without Makefile (manual):

   export VERSION=$(git describe --tags --dirty 2>/dev/null || echo dev)
   export COMMIT=$(git rev-parse --short HEAD 2>/dev/null || echo unknown)
   export DATE=$(date -u '+%Y-%m-%dT%H:%M:%SZ')
   docker compose -f docker-compose.selfhost.yml -f docker-compose.selfhost.build.yml up -d --build
   docker run --rm --entrypoint=/app/multica multica-backend:dev version

Checklist

• I have included a thinking path that traces from project context to this change
• I have run tests locally and they pass
• N/A — I have added or updated tests where applicable
• N/A — If this change affects the UI, I have included before/after screenshots
• N/A — I have updated relevant documentation to reflect my changes
• N/A — If I added a new runtime / coding tool / UI tab, I synced the change to landing copy (apps/web/features/landing/i18n/), starter-content (packages/views/onboarding/utils/starter-content-content-*.ts), and relevant docs (apps/docs/content/docs/)
• N/A — If this PR touches Chinese product copy, I checked it against apps/docs/content/docs/developers/conventions.zh.mdx (terminology, mixed-rule for task / issue / skill)
• I have considered and documented any risks above
• I will address all reviewer comments before requesting merge

AI Disclosure

AI tool used: Claude Code, Multica Agent

Prompt / approach:
Simply ask claude to create an issue in multica to fix version and pass already existing variable from Makefile to docker build.

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