Added .dockerignore, Docker image can be built on arm64. Refs #12

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,20 @@
+# Build output and VCS data are not needed in the image and are huge
+/target/
+/.git/
+node_modules/
+
+# Local configuration with passwords and runtime data must never get into an image layer
+/config.toml
+/data/
+/assets/data/
+/Maildir/
+
+# Local tooling and scratch files
+/.idea/
+/.vscode/
+/.settings/
+/.project
+/.claude/
+/html/
+/local_history.patch
+/end2end/