Remove always pulling latest docker image

Always pulling was proposed and introduced as a way
to ensure that Plugins are always used at their latest version.

However, this does not work if a local Plugin is being developed
that does not yet have a published version at all.

It was decided (by the Memri team) to remove the pulling logic
and instead ask people to always pull the image they need.

src/plugin_run.rs

@@ -140,7 +140,6 @@ fn run_docker_container(
     );
     let args: Vec<String> = vec![
         "run".to_string(),
-        "--pull=always".to_string(),
         format!("--network={}", docker_network),
         format!(
             "--env=POD_FULL_ADDRESS={}",