feat: add GitHub Actions workflow for building and publishing core Docker image

- Introduced a new GitHub Actions workflow in `core-image.yml` to automate the build and publish process for the core Docker image.
- The workflow triggers on pushes to the main branch and supports manual dispatch.
- Configured steps for checking out the repository, setting up Docker Buildx, logging into the GitHub Container Registry, extracting Docker metadata, and building and pushing the Docker image.

Additionally, updated documentation to reflect changes in the deployment process, including a new section for multi-tenant SaaS deployment.

Signed-off-by: Innei <tukon479@gmail.com>

.github/workflows/core-image.yml

@@ -0,0 +1,53 @@
+name: Build & Publish core Docker image
+
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: afilmory/afilmory-core
+
+jobs:
+  build-and-push-core:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract Docker metadata (tags & labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=raw,value=latest
+            type=sha,format=short
+            type=ref,event=branch
+            type=ref,event=tag
+
+      - name: Build and push Docker image (core)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ./Dockerfile.core
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}