Deploy Next.js application on Northflank with DockerFile

Question

I have a problem with env variables and DockerFile. Here's my DockerFile:

FROM node:18-alpine AS base

# Install dependencies only when needed
FROM base AS deps
# Check https://github.com/nodejs/docker- 
node/tree/b4117f9333da4138b03a546ec926ef50a31506c3#nodealpine to understand why libc6-compat might be needed.
RUN apk add --no-cache libc6-compat
WORKDIR /app

# Install dependencies based on the preferred package manager
COPY package.json yarn.lock* package-lock.json* pnpm-lock.yaml* ./
RUN \
  if [ -f yarn.lock ]; then yarn --frozen-lockfile; \
  elif [ -f package-lock.json ]; then npm ci; \
  elif [ -f pnpm-lock.yaml ]; then yarn global add pnpm && pnpm i --frozen-lockfile; \
  else echo "Lockfile not found." && exit 1; \
  fi


# Rebuild the source code only when needed
FROM base AS builder
WORKDIR /app
COPY --from=deps /app/node_modules ./node_modules
COPY . .

# Next.js collects completely anonymous telemetry data about general usage.
# Learn more here: https://nextjs.org/telemetry
# Uncomment the following line in case you want to disable telemetry during the build.
# ENV NEXT_TELEMETRY_DISABLED 1

RUN yarn build

# If using npm comment out above and use below instead
# RUN npm run build

# Production image, copy all the files and run next
FROM base AS runner
WORKDIR /app

ENV NODE_ENV production
# Uncomment the following line in case you want to disable telemetry during runtime.
# ENV NEXT_TELEMETRY_DISABLED 1

RUN addgroup --system --gid 1001 nodejs
RUN adduser --system --uid 1001 nextjs

COPY --from=builder /app/public ./public

# Automatically leverage output traces to reduce image size
# https://nextjs.org/docs/advanced-features/output-file-tracing
COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static

USER nextjs

EXPOSE 3000

ENV PORT 3000

CMD ["node", "server.js"]

Part of the code, I'm getting errors is inside generateStaticParams(), where I am using the env variables, however, I think, that It would have a problem on every static or ssg page, where I fetch the data:

export async function generateStaticParams() {
  const res = await fetch(`${process.env.API_URL}pages`);
  const { docs } = await res.json();

  const result = docs.filter((item: any) => !mainPages.includes(item.slug));

  return result.map((item: any) => ({
    landingPage: item.slug,
  }));
}

An interesting fact, is that I managed to deploy it without any problem on Heroku, and it didn't have any problems with vars, but I really want to deploy it on Northflank.

I have also tried to modify my next config with this:

env: {
    API_URL: process.env.API_URL,
    MY_SECRET_TOKEN: process.env.MY_SECRET_TOKEN
  },
  serverRuntimeConfig: {
    API_URL: process.env.API_URL,
    MY_SECRET_TOKEN: process.env.MY_SECRET_TOKEN
  },

but it didn't help with anything.

Here's the Northflank error:

I have set all envs in northflank, but it doesn't work.