Add shellcheck and mock modules for running unit and linter test (#14432)

#### Why I did it
This is a cherrypick of https://github.com/sonic-net/sonic-buildimage/pull/14062 on 202012 branch

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -27,6 +27,7 @@ RUN apt-get update && apt-get install -y build-essential \
                                          python3-pip \
                                          python3-venv \
                                          rsyslog \
+                                         shellcheck \
                                          snmp \
                                          sshpass \
                                          sudo \
@@ -46,6 +47,7 @@ RUN pip install cffi==1.10.0 \
                 jinja2==2.7.2 \
                 jsonpatch \
                 lxml \
+                mock \
                 natsort \
                 netaddr \
                 netmiko==2.4.2 \