@tk I think I got you a fix.
# 1. I did a fresh setup
./bin/configure
# 2. After finishing the setup we only start the docker container
docker-compose up php
# 3. Switch to a new terminal tab and install the composer deps with the right version of php and wait till it's finished installing.
docker exec -it gnu-social_php_1/usr/bin/php8 /usr/bin/composer.phar update -W
# 4. CTRL+C the terminal in step 2
# 5. Now we should be able to run it
docker-compose up