Update Docker build process and documentation

- Replaced `bun run build:bun` with `npm run build` in Dockerfile for consistency.
- Enhanced `DOCKER.md` to include `DATA_DIR` environment variable usage for database persistence.
- Clarified paths for container and host data storage.

DOCKER.md

@@ -14,6 +14,7 @@ docker build -t 9router .
 docker run --rm \
   -p 20128:20128 \
   -v "$HOME/.9router:/app/data" \
+  -e DATA_DIR=/app/data \
   --name 9router \
   9router
 ```
@@ -23,10 +24,24 @@ The app listens on port `20128` in the container.
 ## What the volume does
 
 ```bash
--v "$HOME/.9router:/app/data"
+-v "$HOME/.9router:/app/data" \
+-e DATA_DIR=/app/data
 ```
 
-This keeps your data outside the container so it survives restarts and image rebuilds.
+`9router` stores its database at `path.join(DATA_DIR, "db.json")`.
+Without `DATA_DIR`, the app falls back to the current user's home directory (for example `~/.9router/db.json` on macOS/Linux). In the container, set `DATA_DIR=/app/data` so the bind mount is actually used.
+
+With the example above, the database file is:
+
+```text
+/app/data/db.json
+```
+
+and it is persisted on the host at:
+
+```text
+$HOME/.9router/db.json
+```
 
 ## Stop container
 
@@ -40,6 +55,7 @@ docker stop 9router
 docker run -d \
   -p 20128:20128 \
   -v "$HOME/.9router:/app/data" \
+  -e DATA_DIR=/app/data \
   --name 9router \
   9router
 ```
@@ -60,6 +76,7 @@ Example:
 docker run --rm \
   -p 20128:20128 \
   -v "$HOME/.9router:/app/data" \
+  -e DATA_DIR=/app/data \
   -e PORT=20128 \
   -e HOSTNAME=0.0.0.0 \
   -e DEBUG=true \

Dockerfile

@@ -12,7 +12,7 @@ RUN --mount=type=cache,target=/root/.npm \
 
 COPY . ./
 ENV NEXT_TELEMETRY_DISABLED=1
-RUN bun run build:bun
+RUN npm run build
 
 FROM oven/bun:1-alpine AS runner
 WORKDIR /app