RStudio Docker running successfully on Debian 12, can't see on the browser at localhost:port

[pathos]

I have a Dockerfile and YAML file to docker compose up on a port.

rstudio4.3.1.Dockerfile:

FROM rocker/rstudio:4.3.1
ENV PASSWORD=password

RUN apt update && apt install libsodium-dev libopenmpi-dev libmpfr-dev openmpi-bin \
  libgsl-dev gsl-bin coinor-clp coinor-libclp-dev gdal-bin libgdal-dev libgeos-dev \
  libudunits2-dev libpoppler-glib-dev libpoppler-cpp-dev -y

RUN install2.r --error --deps TRUE \
  tidytable

# Clean
RUN rm -rf /tmp/downloaded_packages

# Set same logins as the distro
RUN touch /etc/my_passwd /etc/my_shadow && \
  ln -sf /etc/my_passwd /etc/passwd && \
  ln -sf /etc/my_shadow /etc/shadow

CMD ["/init"]

docker-compose.yml

services:
  rstudio4.3.1:
    build:
      context ./
      dockerfile: rstudio4.3.1.Dockerfile
    container_name: rstudio4.3.1
    image: rstudio4.3.1
    ports:
      - 54321:54321
    volumes:
      - /home:/home
    restart: unless-stopped

All of the above seem to be fine as the Docker image seems to be running. It's a fresh Debian install, so no other containers or images are around (also cleaned for retrying). When I go into the browser to localhost:54321 (and trying bunch of other port numbers), Firefox just says 'the connection was reset' about how it is unavialble etc.

Am I missing something? I should be seeing the RStudio log in screen on the browser, then when I log in, I should see the R Studio UI/UX.