Add a note about time taken to build a new Doocker image

README.md

@@ -289,7 +289,7 @@ With this in place, you can run the tests with the following command:
 
 ### Running in a container
 
-The tests can be run in a container, which is requires less setup and will be less likely
+The tests can be run in a container, which requires less setup and will be less likely
 to be affected by your local environment. A Docker Composer file is provided to run the
 tests in a Linux container, against a containerised iRODS server.
 
@@ -298,3 +298,8 @@ To run the tests in a container, you will need to have Docker installed.
 With this in place, you can run the tests with the following command:
 
     docker-compose run app pytest --it
+
+There will be a delay the first time this is run because the Docker image will be built.
+To pre-build the image, you can run:
+
+    docker-compose build