Updated Docker containerizer to set Docker container's resource limits.

This is to ensure the resource limits of Docker container where custom
executor runs can be correctly updated when a new task is launched or
an existing task terminates. And the `resource` field in the `Container`
struct is also renamed to `resourceRequests`.

Review: https://reviews.apache.org/r/72391

include/mesos/values.hpp

@@ -27,6 +27,7 @@ namespace mesos {
 
 std::ostream& operator<<(std::ostream& stream, const Value::Scalar& scalar);
 bool operator==(const Value::Scalar& left, const Value::Scalar& right);
+bool operator!=(const Value::Scalar& left, const Value::Scalar& right);
 bool operator<(const Value::Scalar& left, const Value::Scalar& right);
 bool operator<=(const Value::Scalar& left, const Value::Scalar& right);
 bool operator>(const Value::Scalar& left, const Value::Scalar& right);

src/common/values.cpp

@@ -99,6 +99,12 @@ bool operator==(const Value::Scalar& left, const Value::Scalar& right)
 }
 
 
+bool operator!=(const Value::Scalar& left, const Value::Scalar& right)
+{
+  return !(left == right);
+}
+
+
 bool operator<(const Value::Scalar& left, const Value::Scalar& right)
 {
   return convertToFixed(left.value()) < convertToFixed(right.value());