Ensure that the ECS task definitions only have read-only access to root filesystem

[jspiewak]

[jforest]

We ended up with errors after deploying this to staging.

I think we'll should be able to mount up a temp volume for /opt/R. We need to do some more research to be sure.

For now I've rolled this back, to ensure we're functional

[glin]

Closing because Serverless is no longer used (#261)