Keep cover volume aligned with configured path

backend/Dockerfile

@@ -2,6 +2,7 @@
 
 # --- build stage: compile a static, CGO-free binary ---
 FROM golang:1.26-alpine AS build
+ARG COVER_DIR=/covers
 WORKDIR /src
 
 # Dependencies first for layer caching (changes rarely).
@@ -20,12 +21,13 @@ COPY internal/ ./internal/
 RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags="-s -w" -o /out/server .
 
 # Create the source directory; runtime COPY sets ownership for the named volume.
-RUN mkdir -p /covers
+RUN mkdir -p "$COVER_DIR"
 
 # --- runtime stage: distroless static, non-root ---
 FROM gcr.io/distroless/static:nonroot
+ARG COVER_DIR=/covers
 WORKDIR /
-COPY --from=build --chown=65532:65532 /covers /covers
+COPY --from=build --chown=65532:65532 ${COVER_DIR} ${COVER_DIR}
 COPY --from=build /out/server /server
 EXPOSE 8080
 USER nonroot:nonroot