Fix wrong command examples in build/docker/README.md

[skip ci]

This closes #1883.
diff --git a/build/docker/README.md b/build/docker/README.md
index 7bbf001..08c1372 100644
--- a/build/docker/README.md
+++ b/build/docker/README.md
@@ -109,12 +109,12 @@
 
 To run all unit tests (just like Travis CI does):
 
-    thrift$ dockerrun ubuntu-bionic
+    thrift$ dockerrun thrift/thrift-build:ubuntu-bionic
     root@8caf56b0ce7b:/thrift/src# build/docker/scripts/autotools.sh
 
 To run the cross tests (just like Travis CI does):
 
-    thrift$ dockerrun ubuntu-bionic
+    thrift$ dockerrun thrift/thrift-build:ubuntu-bionic
     root@8caf56b0ce7b:/thrift/src# build/docker/scripts/cross-test.sh
 
 When you are done, you want to clean up occasionally so that docker isn't using lots of extra disk space: