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.

Makefile

@@ -33,10 +33,7 @@ dev:
 	@yarn run build --watch 2>&1
 
 docker:
-	@docker build \
-		--build-arg GIT_COMMIT_SHORT=$$(git rev-parse --short HEAD 2>/dev/null || echo unknown) \
-		--build-arg GIT_COMMIT_FULL=$$(git rev-parse HEAD 2>/dev/null || echo unknown) \
-		-t autistmask .
+	@docker build -t autistmask .
 
 hooks:
 	@echo "Installing pre-commit hook..."