clarify test docker build docs

Author: cornmander

tests/README.md

@@ -2,7 +2,9 @@ The tests verify that `curl-impersonate` has the same network signature as that
 
 ## Running the tests
 
-The tests assume that you've built the `curl-impersonate` docker image before:
+The tests assume that you've built the Debian `curl-impersonate` docker image
+before. See [Building from source](../docs/02_install.md#building-from-source);
+for the Docker-based test image, use:
 
 ```
 docker build -t curl-impersonate -f docker/debian.dockerfile .