build: add pull flag to force image pulling

Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com>

docs/sources/reference/commandline/cli.md

@@ -315,6 +315,7 @@ To kill the container, use `docker kill`.
 
       --force-rm=false     Always remove intermediate containers, even after unsuccessful builds
       --no-cache=false     Do not use cache when building the image
+      --pull=false         Always attempt to pull a newer version of the image
       -q, --quiet=false    Suppress the verbose output generated by the containers
       --rm=true            Remove intermediate containers after a successful build
       -t, --tag=""         Repository name (and optionally a tag) to be applied to the resulting image in case of success