feat: tidy docker setup

Tidy the docker setup and include a Github Action
for publishing docker containers to DockerHub and
Github Container Registry.

Also add a small README file with docker hosting instructions.

.github/workflows/publish.yml

@@ -0,0 +1,40 @@
+name: Publish Docker
+
+on:
+  push:
+    branches: ['release']
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GH_TOKEN }}
+
+      - name: Build the docker image
+        run: ./docker/build.sh
+
+      - name: Push the docker image to DockerHub
+        run: docker push --all-tags documenso/documenso
+
+      - name: Push the docker image to GitHub Container Registry
+        run: docker push --all-tags ghcr.io/documenso/documenso