fix appveyor docker

appveyor.yml

@@ -28,6 +28,9 @@ install:
       $env:TAG_NAME = $env:APPVEYOR_REPO_TAG_NAME.TrimStart("v")
       echo "This is a production deployment for ${env:TAG_NAME}."
     }
+    if("${env:DOCKER_USERNAME}" != "" -and "${env:DOCKER_PASSWORD}" != "") {
+      $env:PUSH_DOCKER = "true"
+    }
     if($isLinux) {
       npm i -g gulp
     } else {
@@ -37,9 +40,8 @@ install:
 
 before_build:
 - sh: |
-    if [ "${DOCKER_USERNAME}" != "" -a "${DOCKER_PASSWORD}" != "" ]
+    if [ "${PUSH_DOCKER}" == "true" ]
     then
-      PUSH_DOCKER = "true"
       echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
     fi
 - ps: |
@@ -95,7 +97,10 @@ build_script:
     rm -rf $STUB_OUTPUT/ca-certificates $STUB_OUTPUT/identity $STUB_OUTPUT/letsencrypt $STUB_OUTPUT/ssl
     rm $STUB_OUTPUT/env/uid.env
     cd docker-stub; zip -r ../docker-stub.zip *; cd ..
-- ps: Push-AppveyorArtifact ./docker-stub.zip
+- ps: |
+    if($isLinux) {
+      Push-AppveyorArtifact ./docker-stub.zip
+    }
 
 
 after_build: