Use md5(1) when available (#3855)

md5sum isn't available by default on macOS. Instead, it ships the
BSD-style md5(1) command, which does the same thing but with different
arguments.

With this change, run-tests works out-of-the-box on macOS.

run-tests

@@ -9,7 +9,11 @@
 image=denseanalysis/ale
 
 # Create docker image tag based on Dockerfile contents
-image_tag=$(md5sum Dockerfile | cut -d' ' -f1)
+if [ -n "$(command -v md5)" ]; then
+    image_tag=$(md5 -q Dockerfile)
+else
+    image_tag=$(md5sum Dockerfile | cut -d' ' -f1)
+fi
 git_version=$(git describe --always --tags)
 
 # Used in all test scripts for running the selected Docker image.