I'd probably setup a container for each companies installation, Keep them all logically seperate. Let them admin it themselves, they've likely all got image/branding, staff, staff groupings and url requirements that make the canned responses bit seem trivial. If you're providing the server, you can also provide support for them, and help them install plugins and setup the system etc, but you don't want to go down the road of literally managing every little thing, delegation!
But that's just me, I'm lazy.
What sort of server are you running it on? How many companies with what volume of staff/users and tickets?
These variables should inform your decision.
But it's a lot easier to break up a containerised system and shard a database that is logically distinct first, than it ever would be after they've all been using it for a year..