Add quotes around pip version

dockers/docker-base/Dockerfile.j2

@@ -52,7 +52,7 @@ COPY ["etc/rsyslog.conf", "/etc/rsyslog.conf"]
 COPY ["etc/rsyslog.d/*", "/etc/rsyslog.d/"]
 COPY ["root/.vimrc", "/root/.vimrc"]
 
-RUN pip install --upgrade pip<21
+RUN pip install --upgrade 'pip<21'
 RUN apt-get purge -y python-pip
 
 # Some Python packages require setuptools (or pkg_resources, which is supplied by setuptools)

dockers/docker-platform-monitor/Dockerfile.j2

@@ -28,7 +28,7 @@ RUN apt-get update &&   \
 
 # TODO: Remove these lines once we no longer need Python 2
 RUN apt-get install -f -y python-dev python-pip
-RUN pip2 install --upgrade pip<21
+RUN pip2 install --upgrade 'pip<21'
 RUN apt-get purge -y python-pip
 
 # On Arista devices, the sonic_platform wheel is not installed in the container.

platform/vs/docker-sonic-vs/Dockerfile.j2

@@ -73,7 +73,7 @@ RUN dpkg -i redis-tools_6.0.6-1~bpo10+1_amd64.deb redis-server_6.0.6-1~bpo10+1_a
 RUN rm redis-tools_6.0.6-1~bpo10+1_amd64.deb redis-server_6.0.6-1~bpo10+1_amd64.deb
 {% endif %}
 
-RUN pip2 install --upgrade pip<21
+RUN pip2 install --upgrade 'pip<21'
 RUN apt-get purge -y python-pip
 RUN pip2 install setuptools==40.8.0
 RUN pip2 install wheel==0.33.6

sonic-slave-buster/Dockerfile.j2

@@ -370,7 +370,7 @@ RUN export VERSION=1.14.2 \
  && rm go$VERSION.linux-*.tar.gz
 
 RUN pip3 install --upgrade pip
-RUN pip2 install --upgrade pip<21
+RUN pip2 install --upgrade 'pip<21'
 RUN apt-get purge -y python-pip python3-pip
 
 # For building Python packages

sonic-slave-stretch/Dockerfile.j2

@@ -339,7 +339,7 @@ RUN export VERSION=1.14.2 \
  && rm go$VERSION.linux-*.tar.gz
 
 RUN pip3 install --upgrade pip
-RUN pip2 install --upgrade pip<21
+RUN pip2 install --upgrade 'pip<21'
 RUN apt-get purge -y python-pip python3-pip
 
 # For p4 build