Well, I opened up the docker target and re-saved it like you said. It re-generated the DB connections like it now works.
When I inspect the DB file that Patrick mentioned it now has:
{
"name": "db",
"module": "dbconnector",
"action": "connect",
"options": {
"client": "postgres",
"connection": {
"host": "db",
"port": 5432,
"user": "db_user",
"password": "xxxxxxx",
"database": "demo"
}
Previously, in this same file it had port 9906.
I will keep an eye on it today and keep switching targets, reloading etc, to see if it continues to work.
If not, i will start a new project and see if i can replicate.