add: Docker image and compose for containerized deploy

Multi-stage build (cgo for the SQLite logger) on debian-slim with ffmpeg,
opus-tools, and ca-certificates bundled. Compose mounts ./data/config.yaml
and ./data/media; SIGTERM cancels in-flight encodes cleanly.

.dockerignore

@@ -0,0 +1,14 @@
+videnc-vibe
+*.mkv
+*.wav
+*.opus
+*.log
+logs.db
+*.json
+/input
+/output
+/originals
+/failed
+/work
+.git
+MANUAL.html