Move MemoryLimit to Execution Environment

spec/factories/execution_environment.rb

@@ -152,7 +152,7 @@ FactoryBot.define do
   end
 
   trait :default_memory_limit do
-    memory_limit { DockerClient::DEFAULT_MEMORY_LIMIT }
+    memory_limit { ExecutionEnvironment::DEFAULT_MEMORY_LIMIT }
   end
 
   trait :default_cpu_limit do

spec/models/execution_environment_spec.rb

@@ -16,7 +16,7 @@ describe ExecutionEnvironment do
   end
 
   it 'validates the minimum value of the memory limit' do
-    execution_environment.update(memory_limit: DockerClient::MINIMUM_MEMORY_LIMIT / 2)
+    execution_environment.update(memory_limit: ExecutionEnvironment::MINIMUM_MEMORY_LIMIT / 2)
     expect(execution_environment.errors[:memory_limit]).to be_present
   end