Limit instance sizes to mitigate Docker timeout failures

* Limit InstanceTypes to specific sizes (xlarge or 2xlarge) so only 1-2 jobs
  can run at a time, instead of potentially 10+. Reduce the number of Docker
  images being pulled at one time to mitigate the 4m Docker image pull
  timeout failures from ECS.
* Restore the r5/r5a instance families for consistency with other serverless
  projects.

serverless-resources.yml

@@ -106,12 +106,22 @@ rBuildsBatchComputeEnvironment:
       InstanceRole:
         "Fn::GetAtt": [ rBuildsIamInstanceProfile, Arn ]
       InstanceTypes:
-        - m6i
-        - m5
-        - m5a
-        - c6i
-        - c5
-        - c5a
+        - m6i.xlarge
+        - m6i.2xlarge
+        - m5.xlarge
+        - m5.2xlarge
+        - m5a.xlarge
+        - m5a.2xlarge
+        - r5.xlarge
+        - r5.2xlarge
+        - r5a.xlarge
+        - r5a.2xlarge
+        - c6i.xlarge
+        - c6i.2xlarge
+        - c5.xlarge
+        - c5.2xlarge
+        - c5a.xlarge
+        - c5a.2xlarge
       Ec2KeyPair: ${self:custom.ec2KeyPair}
       Tags: ${self:provider.stackTags}
       MinvCpus: 0