Simplify Dockerfile

[extremelyonline]

Thank you for creating Offtiktok and dockerizing. I think the image size of the backend image can be reduced by using node:slim

# Use an official Node.js slim base image
FROM node:slim

RUN apt-get update && apt-get install -y --no-install-recommends \
    openssl \
  && rm -rf /var/lib/apt/lists/*

Moreover, I was wondering if the .env file is really necessary? Can't the DATABASE_URL simply be written into the Dockerfile? Perhaps a standard DB name can be given.

FYI, both the frontend and backend image built fine on ARM64!