ci: fix docker cache hash collision

Before this commit the hash used to cache docker images was calculated
from the image's files and the files in the scripts/ directory. This
worked fine when all the files used by an image were in those
directories, but some images pull files from other images, causing hash
collisions in some cases.

This commit changes the hash to include the files of all the docker
images, causing a rebuild of all the images when a single one changes.
That's a bit heavy-handed, but we have no way to track which files an
image pulls in and hash collisions are really painful to deal with.

src/ci/docker/run.sh

@@ -20,9 +20,9 @@ travis_time_start
 if [ -f "$docker_dir/$image/Dockerfile" ]; then
     if [ "$CI" != "" ]; then
       hash_key=/tmp/.docker-hash-key.txt
-      find $docker_dir/$image $docker_dir/scripts -type f | \
-        sort | \
-        xargs cat >> $hash_key
+      rm -f "${hash_key}"
+      echo $image >> $hash_key
+      find $docker_dir -type f | sort | xargs cat >> $hash_key
       docker --version >> $hash_key
       cksum=$(sha512sum $hash_key | \
         awk '{print $1}')