DCP: Allow increasing the pool size when previously empty

app/models/execution_environment.rb

@@ -81,6 +81,8 @@ class ExecutionEnvironment < ApplicationRecord
     runner = Runner.for(author, self)
     output = runner.execute_command(VALIDATION_COMMAND)
     errors.add(:docker_image, "error: #{output[:stderr]}") if output[:stderr].present?
+  rescue Runner::Error::NotAvailable => e
+    Rails.logger.info("The Docker image could not be verified: #{e}")
   rescue Runner::Error => e
     errors.add(:docker_image, "error: #{e}")
   end

lib/runner/strategy/docker_container_pool.rb

@@ -14,9 +14,17 @@ class Runner::Strategy::DockerContainerPool < Runner::Strategy
     FileUtils.mkdir_p(File.expand_path(config[:workspace_root]))
   end
 
-  def self.sync_environment(_environment)
-    # There is no dedicated sync mechanism yet
-    true
+  def self.sync_environment(environment)
+    # There is no dedicated sync mechanism yet. However, we need to emit a warning when the pool was previously
+    # empty for this execution environment. In this case the validation command probably was not executed.
+    return true unless environment.pool_size_previously_changed?
+
+    case environment.pool_size_previously_was
+      when nil, 0
+        false
+      else
+        true
+    end
   end
 
   def self.request_from_management(environment)