ci: automate dev container build via devcontainer.json + workflow (#553)

Closes #552

## Summary
- Add `.devcontainer/devcontainer.json` pointing at `../Dockerfile.dev` so VS Code / Codespaces / any devcontainer-aware tool can build the dev environment directly from source.
- Add `.forgejo/workflows/publish-dev-container.yml` that rebuilds `Dockerfile.dev` and pushes it to `codeberg.org/guettli/sharedinbox-dev` whenever `Dockerfile.dev`, the devcontainer config, or the workflow itself changes on `main`. The image is tagged both `:latest` and with the short commit SHA for pinnable references.
- The workflow uses the built-in `FORGEJO_TOKEN` to log in to Codeberg's container registry — no extra secrets required.

## Notes
- No existing references to `ghcr.io/guettli/sharedinbox-dev` were found in the repo, so issue step 3 (updating image references) is a no-op here.
- `workflow_dispatch` is also enabled so the image can be rebuilt manually if needed.

## Verification
- `python3 -c "import json; json.load(...)"` parses the devcontainer config.
- `python3 -c "import yaml; yaml.safe_load(...)"` parses the workflow.
- Triggers (paths filter) match the source files the issue identifies as drift risks.

Co-authored-by: Thomas Güttler <tilldu@googlemail.com>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/553

.devcontainer/devcontainer.json

@@ -0,0 +1,10 @@
+{
+    "name": "SharedInbox Dev",
+    "build": {
+        "dockerfile": "../Dockerfile.dev",
+        "context": ".."
+    },
+    "workspaceFolder": "/src",
+    "workspaceMount": "source=${localWorkspaceFolder},target=/src,type=bind,consistency=cached",
+    "remoteUser": "ci"
+}

.forgejo/workflows/publish-dev-container.yml

@@ -0,0 +1,44 @@
+name: Publish Dev Container
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'Dockerfile.dev'
+      - '.devcontainer/devcontainer.json'
+      - '.forgejo/workflows/publish-dev-container.yml'
+  workflow_dispatch:
+
+jobs:
+  publish:
+    name: Build & Push sharedinbox-dev
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    env:
+      REGISTRY: codeberg.org
+      IMAGE: codeberg.org/guettli/sharedinbox-dev
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to Codeberg container registry
+        env:
+          FORGEJO_TOKEN: ${{ github.token }}
+        run: |
+          echo "$FORGEJO_TOKEN" \
+            | docker login "$REGISTRY" -u "${{ github.actor }}" --password-stdin
+
+      - name: Build image
+        run: |
+          SHORT_SHA="${GITHUB_SHA:0:7}"
+          docker build \
+            -t "$IMAGE:latest" \
+            -t "$IMAGE:$SHORT_SHA" \
+            -f Dockerfile.dev \
+            .
+
+      - name: Push image
+        run: |
+          SHORT_SHA="${GITHUB_SHA:0:7}"
+          docker push "$IMAGE:latest"
+          docker push "$IMAGE:$SHORT_SHA"