diff --git a/plugins/builder-herokuish/dockerfiles/builder-pre-release.Dockerfile b/plugins/builder-herokuish/dockerfiles/builder-pre-release.Dockerfile
index bef15548b..c43ffea51 100644
--- a/plugins/builder-herokuish/dockerfiles/builder-pre-release.Dockerfile
+++ b/plugins/builder-herokuish/dockerfiles/builder-pre-release.Dockerfile
@@ -1,5 +1,5 @@
 ARG APP_IMAGE
 FROM $APP_IMAGE
 
-ARG DOKKU_APP_USER herokuishuser
+ARG DOKKU_APP_USER=herokuishuser
 COPY --chown=$DOKKU_APP_USER 00-global-env.sh 01-app-env.sh /app/.profile.d/
diff --git a/plugins/builder-herokuish/dockerfiles/builder-release.Dockerfile b/plugins/builder-herokuish/dockerfiles/builder-release.Dockerfile
index f71c47beb..6ce7dbd82 100644
--- a/plugins/builder-herokuish/dockerfiles/builder-release.Dockerfile
+++ b/plugins/builder-herokuish/dockerfiles/builder-release.Dockerfile
@@ -1,7 +1,7 @@
 ARG APP_IMAGE
 FROM $APP_IMAGE
 
-ARG DOKKU_APP_USER herokuishuser
+ARG DOKKU_APP_USER=herokuishuser
 COPY --chown=$DOKKU_APP_USER 00-global-env.sh 01-app-env.sh /app/.profile.d/
 RUN find "/app" \( \! -user "$DOKKU_APP_USER" -o \! -group "$DOKKU_APP_USER" \) -print0 | xargs -0 -r chown "$DOKKU_APP_USER:$DOKKU_APP_USER"
 USER $DOKKU_APP_USER
diff --git a/plugins/builder-herokuish/dockerfiles/copy-source.Dockerfile b/plugins/builder-herokuish/dockerfiles/copy-source.Dockerfile
index 7afe40b99..3fcc43aed 100644
--- a/plugins/builder-herokuish/dockerfiles/copy-source.Dockerfile
+++ b/plugins/builder-herokuish/dockerfiles/copy-source.Dockerfile
@@ -1,7 +1,7 @@
 ARG APP_IMAGE
 FROM $APP_IMAGE
 
-ARG DOKKU_APP_USER herokuishuser
+ARG DOKKU_APP_USER=herokuishuser
 ARG TRACE
 RUN TRACE=$TRACE USER=$DOKKU_APP_USER /exec true
 COPY --chown=$DOKKU_APP_USER . /app
diff --git a/plugins/builder-herokuish/dockerfiles/pre-build.Dockerfile b/plugins/builder-herokuish/dockerfiles/pre-build.Dockerfile
index 7099b46e2..ae115f670 100644
--- a/plugins/builder-herokuish/dockerfiles/pre-build.Dockerfile
+++ b/plugins/builder-herokuish/dockerfiles/pre-build.Dockerfile
@@ -1,6 +1,6 @@
 ARG APP_IMAGE
 FROM $APP_IMAGE
 
-ARG DOKKU_APP_USER herokuishuser
+ARG DOKKU_APP_USER=herokuishuser
 COPY --chown=$DOKKU_APP_USER .env.d /tmp/env
 COPY --chown=$DOKKU_APP_USER .env /app/.env