Build from Dockerfile to improve caching

This should improve build times with changes that don't change the
dependencies in Cargo.toml.

Jenkinsfile

@@ -1,8 +1,8 @@
 pipeline {
   agent {
-    docker {
-      image 'rust:1.55'
-      args '-v "$PWD":/usr/src/image-api'
+    dockerfile {
+      filename 'Dockerfile.ci'
+      args '-v "$PWD:/usr/src/image-api'
     }
   }