Update droid image (Azure#5720)

This reflects the major overhaul of A01 system. The key changes are:
1. Moving metadata from image label to metadata.yml
2. Onboard a01dispatcher executable

scripts/ci/a01/docker_app/metadata.yml

@@ -0,0 +1,17 @@
+kind: DroidMetadata
+version: v3
+product: azurecli
+storage: true
+environments:
+  - name: A01_SP_USERNAME
+    type: secret
+    value: sp.username
+  - name: A01_SP_PASSWORD
+    type: secret
+    value: sp.password
+  - name: A01_SP_TENANT
+    type: secret
+    value: sp.tenant
+  - name: AZURE_TEST_RUN_LIVE
+    type: argument-switch-live
+    value: "True"

scripts/ci/a01/dockerfiles/py36/Dockerfile

@@ -1,23 +1,20 @@
-FROM python:3.6-jessie
+FROM python:3.6-stretch
 
 LABEL a01.product="azurecli"
 LABEL a01.index.schema="v2"
-LABEL a01.env.A01_SP_USERNAME="secret:sp.username"
-LABEL a01.env.A01_SP_PASSWORD="secret:sp.password"
-LABEL a01.env.A01_SP_TENANT="secret:sp.tenant"
-LABEL a01.env.AZURE_TEST_RUN_LIVE="arg-live:True"
-LABEL a01.setting.storage="True"
 
-ADD https://a01tools.blob.core.windows.net/droid/latest/linux/a01droid /app/a01droid 
 COPY build /tmp/build
-COPY docker_app /app
 
 RUN rm /usr/bin/python && ln /usr/local/bin/python3.6 /usr/bin/python && \
-    apt-get update && apt-get install jq && \
-    find /tmp/build -name '*.whl' | xargs pip install && \
-    rm -rf /tmp/build && \
-    python /app/collect_tests.py > /app/test_index && \
-    rm /app/collect_tests.py && \
-    chmod +x /app/a01droid
+    apt-get update && apt-get install -y jq
+
+COPY docker_app/collect_tests.py /tmp/collect_tests.py
+
+RUN find /tmp/build -name '*.whl' | xargs pip install && \
+    mkdir /app && \
+    python /tmp/collect_tests.py > /app/test_index && \
+    rm -rf /tmp
+
+COPY docker_app /app
 
 CMD /app/a01droid

scripts/ci/build_droid.sh

@@ -16,11 +16,11 @@ function title {
 #############################################
 # Clean up artifacts
 title 'Remove artifacts folder'
-if [ -d artifacts ]; then rm -r artifacts; fi
+if [ -d artifacts ] && [ "$AZURE_CLI_BUILD_INCREMENTAL" != "True" ]; then rm -r artifacts; fi
 
 #############################################
 # Build the whl files first
-$dp0/build.sh
+if [ ! -d artifacts ]; then $dp0/build.sh; fi
 
 #############################################
 # Move dockerfile
@@ -30,6 +30,16 @@ cp $dp0/a01/dockerfiles/py36/Dockerfile artifacts/
 # Move other scripts for docker
 cp -R $dp0/a01/* artifacts/
 
+title 'Downloading applications'
+
+curl -L https://a01tools.blob.core.windows.net/droid/latest/linux/a01droid \
+     -o artifacts/docker_app/a01droid
+chmod +x artifacts/docker_app/a01droid
+
+curl -L https://a01tools.blob.core.windows.net/droid/latest/linux/a01dispatcher \
+     -o artifacts/docker_app/a01dispatcher 
+chmod +x artifacts/docker_app/a01dispatcher
+
 #############################################
 # for travis repo slug, remove the suffix to reveal the owner
 # - the offical repo will generate image: azurecli-test-Azure
@@ -42,7 +52,7 @@ image_owner=${image_owner:="private-${USER}"}
 image_owner=`echo $image_owner | tr '[:upper:]' '[:lower:]'`
 version=`cat artifacts/version`
 image_name=azureclidev.azurecr.io/azurecli-test-$image_owner:python3.6-$version
-echo 'Image name: $image_name'
+echo "Image name: $image_name"
 
 title 'Login docker registry'
 if [ $AZURECLIDEV_ACR_SP_USERNAME ] && [ $AZURECLIDEV_ACR_SP_PASSWORD ]; then
@@ -63,4 +73,4 @@ title 'Push docker image as latest'
 if [ "$TRAVIS" == "true" ]; then
     docker tag $image_name azureclidev.azurecr.io/azurecli-test-$image_owner:latest
     docker push azureclidev.azurecr.io/azurecli-test-$image_owner:latest
-fi
+fi