diff --git a/Dockerfile b/Dockerfile
index 6f8aa05..8f544ee 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -4,7 +4,7 @@ FROM $BASE_IMAGE AS builder
 
 # Gems have to be ARG and ENV because they are used as reference in the Gemfile
 ARG RUBYGEM_PUPPET
-ENV RUBYGEM_PUPPET ${RUBYGEM_PUPPET:-8.8.1}
+ENV RUBYGEM_PUPPET=${RUBYGEM_PUPPET:-8.8.1}
 
 ARG RUBYGEM_FACTER
 ENV RUBYGEM_FACTER=${RUBYGEM_FACTER:-4.8.0}