Identify display system and compositor for screen sharing within Docker

I am trying to emulate what x11docker does without using x11docker, and there are helpful samples to do that for Wayland and X.

I had this sample working in Ubuntu 18.04:

Dockerfile

FROM ubuntu:18.04
RUN apt-get update && apt-get install -y chromium-browser
CMD ["/usr/bin/chromium-browser", "--no-sandbox", "--start-fullscreen", "https://duckduckgo.com"]

docker-compose.x11.yml

version: "3"

services:
  app:
    build:
      context: .
      network: host
    environment:
      - DISPLAY=${DISPLAY}
    volumes:
      - /tmp/.X11-unix:/tmp/.X11-unix

I then just ran

docker-compose up --build

and I got a chromium browser pointed to duckduckgo.

However, I have now updated to 20.04 and I can't get this to work any longer with either of the samples.

The sample above generates the following error:

$ docker-compose -f docker-compose.x11.yml up --build
Building app
Sending build context to Docker daemon   5.12kB
Step 1/3 : FROM ubuntu:18.04
 ---> 81bcf752ac3d
Step 2/3 : RUN apt-get update && apt-get install -y chromium-browser
 ---> Using cache
 ---> 2d81deee9198
Step 3/3 : CMD ["/usr/bin/chromium-browser", "--no-sandbox", "--start-fullscreen", "https://duckduckgo.com"]
 ---> Using cache
 ---> 50d1aa785f78
Successfully built 50d1aa785f78
Successfully tagged asdf_app:latest
Starting asdf_app_1 ... done
Attaching to asdf_app_1
app_1  | No protocol specified
app_1  | [1:1:0709/125654.379970:ERROR:browser_main_loop.cc(1402)] Unable to open X display.
asdf_app_1 exited with code 1

So I created an adapted Wayland compose since I understood that is t he default in 20.

docker-compose.wayland.yml

version: "3"

services:
  app:
    build:
      context: .
      network: host
    environment:
      - WAYLAND_DISPLAY=${WAYLAND_DISPLAY}
      - DISPLAY=${DISPLAY}
    volumes:
      - ${XDG_RUNTIME_DIR}/${WAYLAND_DISPLAY}:/tmp/${WAYLAND_DISPLAY}
      - /tmp/.X11-unix:/tmp/.X11-unix

This generates the same error ("Unable to open X display"). My host system does not have any value for WAYLAND_DISPLAY, but does have a value for XDG_RUNTIME_DIR. XDG_SESSION_TYPE has a value of x11. DISPLAY is set to :0.

So what am I actually running here, and how do I find out? Based on what I've found, it seems like I'm running Wayland:

$ cat /etc/gdm3/custom.conf
# GDM configuration storage
#
# See /usr/share/gdm/gdm.schemas for a list of available options.

[daemon]
AutomaticLoginEnable=true
AutomaticLogin=johan

# Uncomment the line below to force the login screen to use Xorg
#WaylandEnable=false

# Enabling automatic login

# Enabling timed login
#  TimedLoginEnable = true
#  TimedLogin = user1
#  TimedLoginDelay = 10

[security]

[xdmcp]

[chooser]

[debug]
# Uncomment the line below to turn on debugging
# More verbose logs
# Additionally lets the X server dump core if it crashes
#Enable=true

So why is XDG_SESSION_TYPE saying x11?

What combination of values do I need to allow me to share my screen with a Docker container?