TT#118659 Switch Dockerfile from Debian buster to bullseye

Change-Id: I986877111b2d931961c1999589cf4a02b64c5d1e

t/Dockerfile

@@ -1,11 +1,11 @@
-# DOCKER_NAME=libtcap-buster
-FROM docker.mgm.sipwise.com/sipwise-buster:latest
+# DOCKER_NAME=libtcap-bullseye
+FROM docker.mgm.sipwise.com/sipwise-bullseye:latest
 
 # Important! Update this no-op ENV variable when this Dockerfile
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT 2021-02-16
+ENV REFRESHED_AT 2021-05-03
 
 RUN apt-get update && \
     apt-get install --assume-yes \
@@ -23,12 +23,12 @@ WORKDIR /code/
 # Instructions for usage
 # ----------------------
 # When you want to build the base image from scratch (jump to the next section if you don't want to build yourself!):
-# % docker build --tag="libtcap-buster" .
-# % docker run --rm -i -t -v $(pwd)/..:/code:rw libtcap-buster:latest bash
+# % docker build --tag="libtcap-bullseye" .
+# % docker run --rm -i -t -v $(pwd)/..:/code:rw libtcap-bullseye:latest bash
 #
 # Use the existing docker image:
-# % docker pull docker.mgm.sipwise.com/libtcap-buster
-# % docker run --rm -i -t -v $(pwd)/..:/code:rw docker.mgm.sipwise.com/libtcap-buster:latest bash
+# % docker pull docker.mgm.sipwise.com/libtcap-bullseye
+# % docker run --rm -i -t -v $(pwd)/..:/code:rw docker.mgm.sipwise.com/libtcap-bullseye:latest bash
 #
 # Inside docker (the command is in history, just press UP button):
 #   ./t/testrunner