Code cleanup and tests

testing/resources/docker_hooks_repo/Dockerfile

@@ -0,0 +1,3 @@
+FROM cogniteev/echo
+
+CMD ["echo", "This is overwritten by the hooks.yaml 'entry'"]

testing/resources/docker_hooks_repo/hooks.yaml

@@ -0,0 +1,11 @@
+-   id: docker-hook
+    name: Docker test hook
+    entry: echo
+    language: docker
+    files: \.txt$
+
+-   id: docker-hook-failing
+    name: Docker test hook with nonzero exit code
+    entry: bork
+    language: docker
+    files: \.txt$