diff --git a/Dockerfile b/Dockerfile
index 0b92433..21473f1 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -4,11 +4,12 @@
 # https://github.com/jlesage/docker-firefox
 #
 
-ARG DOCKER_IMAGE_VERSION=unknown
-
 # Pull base image.
 FROM jlesage/baseimage-gui:alpine-3.9-v3.5.2
 
+# Docker image version is provided via build arg.
+ARG DOCKER_IMAGE_VERSION=unknown
+
 # Define software versions.
 ARG FIREFOX_VERSION=68.0.2-r0
 ARG JSONLZ4_VERSION=c4305b8