Using any port other than 3000 breaks when running nextjs in docker

I have a nextJS project that I'm trying to run on a port other than 3000, more specifically, 8080. When I run the server normally, with the command next dev -p 8080, I can access it at localhost:8080. However, I can't get that to work in Docker.

Here's the Dockerfile:

# Use the official Node.js 18 image as base
# Build stage
FROM node:18-alpine AS build
WORKDIR /app/
COPY ./nextjs/package.json ./nextjs/package-lock.json ./
RUN npm install
COPY ./nextjs .
RUN npm run build

# Run stage
FROM node:18-alpine
WORKDIR /app/
ENV NEXT_TELEMETRY_DISABLED 1
ENV NODE_ENV production

# Copy only the built app from build image
COPY --from=build /app/public ./public
COPY --from=build /app/package.json ./package.json
COPY --from=build /app/.next ./.next
COPY --from=build /app/node_modules ./node_modules

EXPOSE 8080
ENV PORT 8080
CMD ["npm", "run", "start"]

And here are the commands in package.json:

"scripts": {
        "dev": "next dev -p 8080",
        "build": "next build",
        "start": "next start -p 8080",
        "lint": "next lint",
    },

More info:

npx next dev -8080 serves the project in localhost:8080
Running all commands with port 3000, including EXPOSE and ENV in the Dockerfile works.
npx next start -p 8080 also works, the page being accessible in localhost:8080

The only problem is inside a docker container, when using any port other than 3000.

Why is this happening and how can I fix this?

EDIT:

Here are the commands relating to Docker:

docker build -t demo-nextjs:$(TAG) -f Dockerfile.nextjs .

docker run -it demo-nextjs:$(TAG)

Solution

The EXPOSE instruction (docs.docker.com) "[...] doesn't actually publish the port." Thus, if we want to publish the port of a container, we need to specify this with the docker run command.

To publish a port, we use the --publish parameter (docs.docker.com). In our case, we can use:

docker run \
  --publish 8080:8080 \
  --interactive `#same as -i` \
  --tty `#same as -t` \
  demo-nextjs:$(TAG)