refactor: derive git info inside Docker instead of --build-arg

- Remove .git from .dockerignore so it's available in Docker build context
- Install git in Docker image (apt-get)
- Remove ARG/ENV GIT_COMMIT_SHORT/FULL from Dockerfile
- Remove --build-arg from Makefile docker target
- Simplify build.js to use git CLI directly (no env var indirection)

build.js already had fallback logic to shell out to git; now that .git
is present in the build context, it works directly without needing
values passed in from outside Docker.

Dockerfile

@@ -1,7 +1,7 @@
 # node:22-slim (22.x LTS), 2026-02-24
 FROM node@sha256:5373f1906319b3a1f291da5d102f4ce5c77ccbe29eb637f072b6c7b70443fc36
 
-RUN apt-get update && apt-get install -y --no-install-recommends make && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y --no-install-recommends make git && rm -rf /var/lib/apt/lists/*
 RUN corepack enable && corepack prepare yarn@1.22.22 --activate
 
 WORKDIR /app
@@ -11,10 +11,5 @@ RUN yarn install --frozen-lockfile
 
 COPY . .
 
-ARG GIT_COMMIT_SHORT=unknown
-ARG GIT_COMMIT_FULL=unknown
-ENV GIT_COMMIT_SHORT=${GIT_COMMIT_SHORT}
-ENV GIT_COMMIT_FULL=${GIT_COMMIT_FULL}
-
 RUN make check
 RUN make build