Add stack.yaml file to containers.

`stack setup` fails due to ChasingBottoms missing from Stackage

We default to lts-9.20 because lts-10.0 is currently not supported by many
development tools (such as intero) because ghc-mod does not yet support GHC
8.2 (DanielG/ghc-mod#900).

This was tested and works with resolvers lts-10.0 and lts-9.20.

Using older resolvers <=lts-7.24 causes a conflict with the package ranges
specified in containers.cabal.

stack.yaml

@@ -0,0 +1,11 @@
+flags: {}
+packages:
+- '.'
+
+### Uncoment the resolver you want to use and re-run `stack build/test/bench`.
+# resolver: lts-10.0
+resolver: lts-9.20
+
+### ChasingBottoms is only in Stackage snapshots lts-7.24 and below.
+extra-deps:
+- ChasingBottoms-1.3.1.3