chore(docker): switch to node:22-bookworm-slim and add .env.example

- Dockerfile base now node:22-bookworm-slim (glibc) so better-sqlite3
  uses prebuilt binaries and node-gyp/python build deps are no longer
  required. Also satisfies @discordjs/voice node>=22.12 engine req.
- Drop redundant `mkdir -p dist` (build script handles it).
- Add .env.example covering TOKEN, APPID, PREFIX, DBPATH, GUILDID,
  CHZZK_NID_AUT/SES, SIGNATURE_HOST, TTSPATH, DEV, DEBUG, REPLACETEXT.
- README: update base image note.

Dockerfile

@@ -1,6 +1,8 @@
-FROM node:20-alpine
+FROM node:22-bookworm-slim
 
-RUN apk add --no-cache ffmpeg
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends ffmpeg \
+    && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app
 
@@ -10,8 +12,6 @@ RUN npm install
 
 COPY . .
 
-RUN mkdir -p dist
-
 RUN npm run build
 
 CMD ["npm", "run", "start"]