[Identity] Update integration test Dockerfiles (#36480)

Ensure that we are pulling images from MCR so that we do not hit rate
limits.

Signed-off-by: Paul Van Eck <paulvaneck@microsoft.com>

Author: pvaneck

sdk/identity/azure-identity/tests/integration/azure-functions/Dockerfile

@@ -4,11 +4,10 @@
 # ------------------------------------
 # docker can't tell when the repo has changed and will therefore cache this layer
 
-# internal users should provide MCR registry to build via 'docker build . --build-arg REGISTRY="mcr.microsoft.com/mirror/docker/library/"'
-# public OSS users should simply leave this argument blank or ignore its presence entirely
-ARG REGISTRY=""
+# public OSS users should make this an empty string: 'docker build . --build-arg REGISTRY=""'
+ARG REGISTRY="mcr.microsoft.com/mirror/docker/library/"
 
-FROM ${REGISTRY}alpine:3.19 as repo
+FROM ${REGISTRY}alpine:3.16 AS repo
 RUN apk --no-cache add git
 RUN git clone https://github.com/Azure/azure-sdk-for-python --single-branch --depth 1 /azure-sdk-for-python
 

sdk/identity/azure-identity/tests/integration/azure-kubernetes-service/Dockerfile

@@ -4,11 +4,10 @@
 # ------------------------------------
 # docker can't tell when the repo has changed and will therefore cache this layer
 
-# internal users should provide MCR registry to build via 'docker build . --build-arg REGISTRY="mcr.microsoft.com/mirror/docker/library/"'
-# public OSS users should simply leave this argument blank or ignore its presence entirely
-ARG REGISTRY=""
+# public OSS users should make this an empty string: 'docker build . --build-arg REGISTRY=""'
+ARG REGISTRY="mcr.microsoft.com/mirror/docker/library/"
 
-FROM ${REGISTRY}alpine:3.19 as repo
+FROM ${REGISTRY}alpine:3.16 AS repo
 RUN apk --no-cache add git
 RUN git clone https://github.com/Azure/azure-sdk-for-python --single-branch --depth 1 /azure-sdk-for-python