message: [docker] move to ubuntu 20.04
diff: # POSSIBILITY OF SUCH DAMAGE.
  #
  -FROM ubuntu:bionic
  +FROM ubuntu:focal
  ARG OT_BACKBONE_CI
  ARG OTBR_OPTIONS
  @@ -61,7 +61,7 @@ ENV OTBR_BUILD_DEPS apt-utils build-essential psmisc ninja-build cmake wget ca-c
  libavahi-client-dev libboost-dev libboost-filesystem-dev libboost-system-dev libjsoncpp-dev
  # Required for OpenThread Backbone CI
  -ENV OTBR_OT_BACKBONE_CI_DEPS curl ca-certificates
  +ENV OTBR_OT_BACKBONE_CI_DEPS curl
  # Required and installed during build (script/bootstrap) when RELEASE=1, could be removed
  ENV OTBR_NORELEASE_DEPS \
  @@ -71,6 +71,7 @@ ENV OTBR_NORELEASE_DEPS \
  RUN apt-get update \
  && apt-get install --no-install-recommends -y $OTBR_DOCKER_REQS $OTBR_DOCKER_DEPS \
  + && ([ "${OT_BACKBONE_CI}" != "1" ] || apt-get install --no-install-recommends -y $OTBR_OT_BACKBONE_CI_DEPS) \
  && ln -fs /usr/share/zoneinfo/UTC /etc/localtime \
  && ./script/bootstrap \
  && ./script/setup \
  @@ -86,8 +87,7 @@ RUN apt-get update \
  && apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false $OTBR_BUILD_DEPS \
  && ([ "${RELEASE}" = 1 ] || apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false "$OTBR_NORELEASE_DEPS";) \
  && rm -rf /var/lib/apt/lists/* \
  - ) \
  - && [ "${OT_BACKBONE_CI}" != "1" ] || apt-get install --no-install-recommends -y $OTBR_OT_BACKBONE_CI_DEPS
  + )
  ENTRYPOINT ["/app/etc/docker/docker_entrypoint.sh"]
  