I have a next.js application that requires environment variables, I have included the .env file itself along with adding the variables to beanstalk environment, still the variables are passed to docker host. I use CodePipeline to build and upload to ECR and then deploy to EB

DockerFile

# Base stage to install node deps
FROM --platform=linux/amd64 node:20-alpine AS base
WORKDIR /app

RUN apk add --no-cache libc6-compat

COPY package*.json ./
COPY . ./

RUN npm ci

# Build stage to transpile `src` into `dist`

FROM base AS build

COPY --from=base package*.json ./
COPY --from=base /app/node_modules ./node_modules
COPY . .

RUN npm run build 
    && npm prune --production

# Final stage for production app image

FROM base AS production

ENV NODE_ENV="production"
ENV PORT=80

COPY --from=build --chown=node:node package*.json ./
COPY --from=build --chown=node:node /app/node_modules ./node_modules

# Remove if you don't have public files
COPY --from=build --chown=node:node /app/public ./public
COPY --from=build --chown=nextjs:nodejs /app/.next ./.next
RUN mkdir -p /app/shared/public

EXPOSE $PORT

CMD ["npm", "start"]

Docker-Compose

services:
  web:
    image: image
    platform: linux/amd64
    restart: always
    ports:
      - 3000:3000
    command:
      - /bin/sh
      - -c
      - |
        # Sync shared public files
        rm -rf /app/shared/public/*
        cp -r /app/public/* /app/shared/public
        #
        # You can add more commands before startup: compile assets, run migrations, ... if you have
        # ...
        #
        # Start node app
        npm run start
    environment:
      - DB_NAME=${DB_NAME}
      - DB_HOSTNAME=${DB_HOSTNAME}
      
    volumes:
      - public_assets:/app/shared/public
      - /app/node_modules
      - /app/.next

  nginx:
    image: nginx:alpine
    platform: linux/amd64
    restart: always
    ports:
      - 80:80
    depends_on:
      - web
    volumes:
      - public_assets:/app/public
      - ./nginx.conf:/etc/nginx/conf.d/default.conf
      - /app/node_modules
      - /app/.next

volumes:
  public_assets:

my buildspec for CodeBuild

version: 0.2

phases:
  pre_build:
    commands:
      - echo "Logging in to Amazon ECR..."
      - aws ecr get-login-password --region $AWS_DEFAULT_REGION | docker login --username AWS --password-stdin $AWS_ACCOUNT_ID.dkr.ecr.$AWS_DEFAULT_REGION.amazonaws.com
      - COMMIT_SHA=$(echo $CODEBUILD_RESOLVED_SOURCE_VERSION | cut -c 1-7)
      - IMAGE_TAG=${COMMIT_SHA:-latest}
  build:
    commands:
      - echo "Build started on `date`"
      - docker build -t $IMAGE_REPOSITORY:$IMAGE_TAG .
      - docker tag $IMAGE_REPOSITORY:$IMAGE_TAG $AWS_ACCOUNT_ID.dkr.ecr.$AWS_DEFAULT_REGION.amazonaws.com/$IMAGE_REPOSITORY:$IMAGE_TAG
      - echo "Build completed on `date`"
  post_build:
    commands:
      - echo "Push the docker image to AWS ECR..."
      - docker push $AWS_ACCOUNT_ID.dkr.ecr.$AWS_DEFAULT_REGION.amazonaws.com/$IMAGE_REPOSITORY:$IMAGE_TAG
      - echo "Update IMAGE_TAG in docker-compose.yml..."
      - sed -i -e "s/${IMAGE_TAG:-latest}/$IMAGE_TAG/g" docker-compose.yml
artifacts:
  files:
    - .ebextensions/**/*
    - docker-compose.yml
    - nginx.conf
cache:
  paths:
    - node_modules