Added --purge of base docker image packages before installing new one…

…s. (sonic-net#819)

Why I did:
Swss debian package used in VS docker is not the same artifact as generated by Azure pipeline
Compared md5sum of binaries of Azure artifact swss debian package and the one running in VS docker and they are not matching.

How I did:
To fix this doing explicit purge all packages before re-installing.

.azure-pipelines/docker-sonic-vs/Dockerfile

@@ -4,6 +4,15 @@ ARG docker_container_name
 
 ADD ["debs", "/debs"]
 
+RUN dpkg --purge python-swsscommon
+RUN dpkg --purge python3-swsscommon
+RUN dpkg --purge swss
+RUN dpkg --purge libsairedis
+RUN dpkg --purge libswsscommon
+RUN dpkg --purge libsaimetadata
+RUN dpkg --purge libsaivs
+RUN dpkg --purge syncd-vs
+
 RUN dpkg -i /debs/libswsscommon_1.0.0_amd64.deb
 RUN dpkg -i /debs/python-swsscommon_1.0.0_amd64.deb
 RUN dpkg -i /debs/python3-swsscommon_1.0.0_amd64.deb