add imagemagick in ci docker image

.gitlab-ci.yml

@@ -24,7 +24,7 @@ cache:
 test-sbt:
   extends: .test-sbt
   before_script:
-    - apt-get update --allow-releaseinfo-change && apt-get install -y ffmpeg
+    - apt-get update --allow-releaseinfo-change && apt-get install -y ffmpeg imagemagick
 assembly-sbt:
   extends: .assembly-sbt