Fix: Use Coolify's SOURCE_COMMIT instead of git detection

Research findings:
- Coolify provides SOURCE_COMMIT environment variable with git commit hash
- Must enable "Include Source Commit in Build" in Coolify General settings
- Disabled by default to preserve Docker build cache
- Coolify automatically passes SOURCE_COMMIT as build arg

Changes:
1. Dockerfile: Use SOURCE_COMMIT build arg instead of GIT_COMMIT
2. Dockerfile: Remove git installation and detection logic
3. Dockerfile: Extract first 7 chars of SOURCE_COMMIT for short hash
4. docker-compose.yml: Pass SOURCE_COMMIT instead of GIT_COMMIT
5. docker-compose.yml: Remove GIT_COMMIT from env (set in Dockerfile)
6. README: Add instructions to enable "Include Source Commit in Build"
7. README: Update version tracking documentation

How it works:
- Coolify passes SOURCE_COMMIT to docker build
- Dockerfile takes first 7 characters: ${SOURCE_COMMIT:0:7}
- Sets as GIT_COMMIT env var for runtime
- Health endpoint returns the short commit ID

Note: Returns "unknown" if SOURCE_COMMIT tracking is not enabled.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Dockerfile

@@ -1,13 +1,13 @@
 FROM node:18-alpine
 
-# Build arguments for versioning
-ARG GIT_COMMIT=unknown
+# Build arguments - Coolify provides SOURCE_COMMIT automatically
+ARG SOURCE_COMMIT=unknown
 ARG APP_VERSION=1.0.0
 
 WORKDIR /app
 
-# Install curl for healthcheck and git for commit info
-RUN apk add --no-cache curl git
+# Install curl for healthcheck
+RUN apk add --no-cache curl
 
 # Copy package files first for layer caching
 COPY package*.json ./
@@ -16,14 +16,9 @@ RUN npm install --production
 # Copy app code (changes more often — below the cached layer)
 COPY . .
 
-# Try to get short git commit (7 chars) if not provided and .git exists
-RUN if [ "$GIT_COMMIT" = "unknown" ] && [ -d .git ]; then \
-      GIT_COMMIT=$(git rev-parse --short HEAD 2>/dev/null || echo "unknown"); \
-    fi && \
-    echo "export GIT_COMMIT=${GIT_COMMIT}" >> /etc/profile.d/git-version.sh
-
 # Set environment variables for runtime
-ENV GIT_COMMIT=${GIT_COMMIT} \
+# Extract first 7 characters of SOURCE_COMMIT for short hash
+ENV GIT_COMMIT=${SOURCE_COMMIT:0:7} \
     APP_VERSION=${APP_VERSION}
 
 EXPOSE 3000