docs: add example usage for image_build

docs/image_build.md

@@ -3,17 +3,102 @@
 ## Inputs
 
 <!-- AUTO-DOC-INPUT:START - Do not remove or modify this section -->
+
 No inputs.
+
 <!-- AUTO-DOC-INPUT:END -->
 
 ## Outputs
 
 <!-- AUTO-DOC-OUTPUT:START - Do not remove or modify this section -->
+
 No outputs.
+
 <!-- AUTO-DOC-OUTPUT:END -->
 
 ## Secrets
 
 <!-- AUTO-DOC-SECRETS:START - Do not remove or modify this section -->
+
 No secrets.
+
 <!-- AUTO-DOC-SECRETS:END -->
+
+## Example Usage
+
+**Standalone**
+
+```yaml
+backend-build:
+  uses: hotosm/gh-workflows/.github/workflows/image_build.yml
+  with:
+    context: src/backend
+    build_target: prod
+    image_tags: |
+      "ghcr.io/hotosm/fmtm/backend:latest"
+    build_args: |
+      APP_VERSION=0.1.0
+```
+
+**Passing Variables**
+
+Example variable extraction from FMTM:
+
+```yaml
+name: Extract Project Variables
+
+on:
+  workflow_call:
+    inputs:
+      environment:
+        description: "The GitHub environment to extract vars from."
+        type: string
+        default: ""
+    outputs:
+      api_version:
+        description: "Backend API Version."
+        value: ${{ jobs.extract-vars.outputs.api_version }}
+
+jobs:
+  extract-vars:
+    runs-on: ubuntu-latest
+    environment: ${{ inputs.environment }}
+    outputs:
+      api_version: ${{ steps.extract_api_version.outputs.api_version }}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      - name: Extract api version
+        id: extract_api_version
+        run: |
+          cd src/backend
+          API_VERSION=$(python -c 'from app.__version__ import __version__; print(__version__)')
+          echo "api_version=${API_VERSION}" >> $GITHUB_OUTPUT
+```
+
+Then example variable passing from FMTM:
+
+```yaml
+jobs:
+  extract-vars:
+    needs:
+      - pytest
+      - frontend-tests
+    uses: ./.github/workflows/extract_vars.yml
+    with:
+      environment: ${{ github.ref_name }}
+
+  backend-build:
+    uses: hotosm/gh-workflows/.github/workflows/image_build.yml
+    needs: extract-vars
+    with:
+      context: src/backend
+      build_target: prod
+      image_tags: |
+        "ghcr.io/hotosm/fmtm/backend:${{ needs.extract-vars.outputs.api_version }}-${{ github.ref_name }}"
+        "ghcr.io/hotosm/fmtm/backend:latest"
+      build_args: |
+        APP_VERSION=${{ needs.extract-vars.outputs.api_version }}
+```