Bug Description

When running the Docker image with a non-default BACKEND_PORT (e.g., 3781 instead of 8001), the frontend always throws NEXT_PUBLIC_API_BASE placeholder was not substituted in the browser console. The settings page shows NEXT_PUBLIC_API_BASE is not configured, and sessions are not displayed.

Root Cause

The Dockerfile builds the Next.js frontend with a placeholder value for NEXT_PUBLIC_API_BASE:

# Dockerfile, line 47 (frontend-builder stage)
RUN echo "NEXT_PUBLIC_API_BASE=__NEXT_PUBLIC_API_BASE_PLACEHOLDER__" > .env.local

At build time, Turbopack inlines process.env.NEXT_PUBLIC_API_BASE into the JS bundle. The placeholder appears in two contexts:

1. Runtime value location -- resolveBase() uses the inlined string as the API URL
2. Validation guard location -- Turbopack injects a check: if(!e || e === "__NEXT_PUBLIC_API_BASE_PLACEHOLDER__") throw ...

At container startup, start-frontend.sh (line 266-267) runs:

find /app/web/.next -type f \( -name "*.js" -o -name "*.json" \) -exec \
    sed -i "s|__NEXT_PUBLIC_API_BASE_PLACEHOLDER__|${API_BASE}|g" {} \;

This replaces all occurrences of the placeholder -- including the comparison constant inside the validation guard. After sed:

• resolveBase() correctly returns http://localhost:${BACKEND_PORT} (correct)
• The validation guard becomes if(!e || e === "http://localhost:3781") -- always true -> always throws (broken)

Additional Issue: Hardcoded http://localhost:8001

Some built modules reference http://localhost:8001 directly (the build-time default port from BACKEND_PORT=8001) instead of using the placeholder, so the sed above does not reach them. These modules keep pointing to the wrong port.

Reproduction Steps

1. Build Docker image (default BACKEND_PORT=8001)
2. Run container with e.g. BACKEND_PORT=3781 (or any port != 8001)
3. Open frontend in browser -> console error: placeholder was not substituted
4. Settings page shows: NEXT_PUBLIC_API_BASE is not configured
5. Sessions list is empty

Impact

Any user running on a non-default port is affected. This is particularly common on Windows where ports 7930-8129 are often reserved by the system (netsh excluded port range), making the default 8001/8002 ports unusable without manual reconfiguration.

Proposed Fix

Add two additional sed commands after the existing placeholder replacement in start-frontend.sh (after line 267):

# 1. Fix hardcoded default port 8001 in modules that don't use the placeholder
find /app/web/.next -type f -name "*.js" -exec \
    sed -i "s|http://localhost:8001|${API_BASE}|g" {} \;

# 2. Fix validation guard by breaking the identity comparison (client chunks only)
#    The validation check uses `let t="<API_BASE>"` and compares with `e === t`.
#    After sed replaces the placeholder, both sides equal the actual URL, so
#    the check always fires. Breaking the comparison variable prevents this.
find /app/web/.next/static/chunks -type f -name "*.js" -exec \
    sed -i "s|let t=\"${API_BASE}\"|let t=\"__VALIDATED__\"|g" {} \; 2>/dev/null || true

Note: The second fix targets only static/chunks/ (client-side bundles). The validation guard does not exist in server-side (SSR) chunks.

Alternative Long-term Fix

Use Turbopack's define configuration to replace process.env.NEXT_PUBLIC_API_BASE at build time with a runtime expression, eliminating the need for post-build sed substitution entirely.

Workaround (manual, inside running container)

find /app/web/.next -name '*.js' -exec sed -i 's|http://localhost:8001|http://localhost:3781|g' {} +
find /app/web/.next/static/chunks -name '*.js' -exec sed -i 's|let t="http://localhost:3781"|let t="__VALIDATED__"|g' {} +

Environment

• Host OS: Windows (Docker Desktop)
• Container: DeepTutor Docker image v1.3.6 (ghcr.io/hkuds/deeptutor:latest)
• Dockerfile: multi-stage (Node 22-slim frontend builder + Python 3.11-slim runtime)