I have a nextjs app and a dotnet8 app running in docker with one docker-compose file. Here are my files:

The nextjs/Dockerfile:

FROM node:20.15.0-alpine3.20 AS base

FROM base AS deps
RUN apk add --no-cache libc6-compat

WORKDIR /app

COPY --from=ext_path somefile somefile

COPY package*.json ./
RUN mkdir node_modules
RUN npm install --no-audit --omit=dev
RUN npm install sharp

FROM base AS builder
WORKDIR /app
COPY --from=deps /app/node_modules ./node_modules
COPY --from=deps /app/.env.local ./.env.local
COPY . .

ENV NEXT_TELEMETRY_DISABLED 1

RUN npm run build

FROM base AS runner
WORKDIR /app

ENV NODE_ENV production

ENV NEXT_TELEMETRY_DISABLED 1

COPY --from=deps /app/.env.local ./.env.local
COPY --from=ext_path somefile somefile

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

COPY --from=builder /app/public ./public
COPY --from=builder /app/package.json ./package.json
COPY --from=builder /app/next.config.mjs ./next.config.mjs

COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static

RUN mkdir -p ./public/v1/uploads
RUN chown nextjs:nodejs ./public/v1/uploads

USER nextjs

EXPOSE 80

ENV PORT 80
ENV HOSTNAME "0.0.0.0"

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

And the dotnet8/Dockerfile:

FROM mcr.microsoft.com/dotnet/aspnet:8.0-alpine AS base
WORKDIR /app

EXPOSE 80
ENV ASPNETCORE_URLS=http://+:80

USER app
FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/sdk:8.0-alpine AS build
ARG configuration=Release

COPY ["MyApp.sln", "MyApp.sln"]
COPY ["MyApp/MyApp.csproj", "MyApp/"]

RUN dotnet restore "MyApp.sln"

COPY MyApp/ MyApp/

WORKDIR "/MyApp"

FROM build AS publish
ARG configuration=Release
RUN dotnet publish "MyApp.csproj" -c $configuration -o /app/publish /p:UseAppHost=false

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "MyApp.dll"]

The production/docker.env file:

SRV_NAME=myproject

DOCKER_ENV=production
RUNTIME_ENV=production

WEB_EXPOSE_PORT=4000
API_EXPOSE_PORT=4100

UPLOAD_MOUNT_PATH=/var/lib/MyApp/web/public/v1/uploads

And finally the docker-compose.yml file:

networks:
    default:
        name: ${SRV_NAME}_default
        external: false
    infra:
        name: infra
        external: true
        driver: bridge

services:
    my-web:
        container_name: ${SRV_NAME}-web
        image: jd/my-web:latest
        hostname: ${SRV_NAME}-web
        depends_on:
            - my-api
        build:
            context: ./nextjs
            dockerfile: ./Dockerfile
            network: host
            args:
                NODE_ENV: ${RUNTIME_ENV}
            additional_contexts:
                ext_path: ./${DOCKER_ENV}/ # This is just for reading some additional resources.
        volumes:
            - ${UPLOAD_MOUNT_PATH}:/app/public/v1/uploads:rw
        environment:
            - NODE_ENV=${RUNTIME_ENV}
        ports:
            - ${WEB_EXPOSE_PORT}:80
        networks:
            default:
            infra:

    my-api:
        container_name: ${SRV_NAME}-api
        image: jd/my-api:latest
        hostname: ${SRV_NAME}-api
        build:
            context: ./dotnet8
            dockerfile: ./MyApp/Dockerfile
        volumes:
            - ${UPLOAD_MOUNT_PATH}:/app/public/v1/uploads:rw
        ports:
            - ${API_EXPOSE_PORT}:80
        networks:
            default:
            infra:

So, I’m trying to save an uploaded file from nextjs app. But I’m getting this error:

Error: EACCES: permission denied, open '/app/public/v1/uploads/639ceeac-65f3-4726-8a31-9e55e603ff9b.png'
    at async open (node:internal/fs/promises:639:25)
    at async writeFile (node:internal/fs/promises:1219:14)
    at async c (/app/.next/server/chunks/106.js:1:21544)
    at async /app/node_modules/next/dist/compiled/next-server/app-page.runtime.prod.js:16:418
    at async rP (/app/node_modules/next/dist/compiled/next-server/app-page.runtime.prod.js:15:7978)
    at async r9 (/app/node_modules/next/dist/compiled/next-server/app-page.runtime.prod.js:18:1139)
    at async doRender (/app/node_modules/next/dist/server/base-server.js:1438:30)
    at async cacheEntry.responseCache.get.routeKind (/app/node_modules/next/dist/server/base-server.js:1599:28)
    at async NextNodeServer.renderToResponseWithComponentsImpl (/app/node_modules/next/dist/server/base-server.js:1507:28)
    at async NextNodeServer.renderPageComponent (/app/node_modules/next/dist/server/base-server.js:1931:24) {
  errno: -13,
  code: 'EACCES',
  syscall: 'open',
  path: '/app/public/v1/uploads/639ceeac-65f3-4726-8a31-9e55e603ff9b.png'
}

So have you any idea what’s wrong here? And how to solve the problem? Thanks in advance.