Make Docker image easier to use (#50)

* Use single server example as default frontend

This just works out of the box

* Refactor Dockerfile

- Remove the asset dir since it's embedded in the executable anyway
- Use the code in the current directory to build instead of downloading.
  Makes local testing very easy.
- Cache Go dependencies
- Run without root privileges. Note that using a BoltDB database
  requires mounting a volume with the correct permissions. The default
  database has thus been changed to memory so that starting the image
  just works.

* Automatically publish images

.github/workflows/docker-publish.yml

@@ -0,0 +1,56 @@
+name: Docker
+on:
+  push:
+    branches: [ "master" ]
+    # Publish semver tags as releases.
+    tags: [ 'v*.*.*' ]
+  pull_request:
+    branches: [ "master" ]
+
+env:
+  # Use docker.io for Docker Hub if empty
+  REGISTRY: ghcr.io
+  # github.repository as <account>/<repo>
+  IMAGE_NAME: ${{ github.repository }}
+
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      # Login against a Docker registry except on PR
+      # https://github.com/docker/login-action
+      - name: Log into registry ${{ env.REGISTRY }}
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@28218f9b04b4f3f62068d7b6ce6ca5b26e35336c
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      # Extract metadata (tags, labels) for Docker
+      # https://github.com/docker/metadata-action
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@98669ae865ea3cffbcbaa878cf57c20bbf1c6c38
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+
+      # Build and push Docker image with Buildx (don't push on PR)
+      # https://github.com/docker/build-push-action
+      - name: Build and push Docker image
+        id: build-and-push
+        uses: docker/build-push-action@ac9327eae2b366085ac7f6a2d02df8aa8ead720a
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}