Verify that downloaded plugin is really a Zip archive
diff --git a/Dockerfile b/Dockerfile
index 2d687f0..64d1fe3 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,6 +1,6 @@
 FROM java:8-jdk
 
-RUN apt-get update && apt-get install -y wget git curl zip && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y wget git curl zip file && rm -rf /var/lib/apt/lists/*
 
 ENV JENKINS_HOME /var/jenkins_home
 ENV JENKINS_SLAVE_AGENT_PORT 50000