fix(deploy): app-owned cache/books dirs in image + .dockerignore

.dockerignore

@@ -0,0 +1,5 @@
+.env
+.git
+library/
+.superpowers/
+backend/booklib*

deploy/Dockerfile.api

@@ -8,6 +8,7 @@ RUN CGO_ENABLED=0 go build -trimpath -o /server ./cmd/server
 FROM alpine:3.20
 RUN adduser -D -H app
 COPY --from=build /server /server
+RUN mkdir -p /data/cache /data/books && chown app:app /data/cache /data/books
 USER app
 EXPOSE 8080
 ENTRYPOINT ["/server"]