Trouble Setting Up WebIDE Terminal

So, I’m trying to set up the Web IDE terminal but when I try to open it I get “An unexpected error occurred while starting the Web Terminal.”. So far I’ve got the following:

.gitlab-webide.yml

terminal:
   image: mcr.microsoft.com/dotnet/core/sdk:3.1
   before_script:
     - apt-get update
   script: sleep 60

runner config.toml

terminal:

concurrent=1
check_interval = 0
[session_server]
   listen_address = "0.0.0.0:8093"
   advertise_address = "rgg-runner-1.internaldomain.internal:8093"
   session_timeout = 1800
[[runners]]
...

I access gitlab through an edge reverse proxy accessible at https://gitlab.mydomain.co.uk. I’ve got the upgrade and connection headers set.

Any ideas?