Fix Dockerfile syntax

Change-Id: I3ebf670402d23beb5f6ea974454a6245369e670f
diff --git a/Dockerfile b/Dockerfile
index 417932e..3adc1c6 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM metacpan/metacpan-api as builder
+FROM metacpan/metacpan-api AS builder
 
 # Copy repository respecting .dockerignore
 COPY . /derekovecs