message: [dockerfile] add more ARGs
  This commit adds more ARG to Dockerfile so as to allow building
  customized Docker images.
diff: @@ -29,18 +29,23 @@ FROM ubuntu:bionic
  ARG OT_BACKBONE_CI
  ARG OTBR_OPTIONS
  +ARG DNS64
  +ARG NAT64
  ARG REFERENCE_DEVICE
  +ARG RELEASE
  -ENV OT_BACKBONE_CI=${OT_BACKBONE_CI}
  +ENV OT_BACKBONE_CI=${OT_BACKBONE_CI:-0}
  ENV OTBR_OPTIONS=${OTBR_OPTIONS}
  ENV DEBIAN_FRONTEND noninteractive
  ENV PLATFORM ubuntu
  -ENV REFERENCE_DEVICE=${REFERENCE_DEVICE}
  -ENV RELEASE 1
  -ENV NAT64 1
  -ENV DNS64 1
  +ENV REFERENCE_DEVICE=${REFERENCE_DEVICE:-0}
  +ENV RELEASE=${RELEASE:-1}
  +ENV NAT64=${NAT64:-1}
  +ENV DNS64=${DNS64:-1}
  ENV DOCKER 1
  +RUN env
  +
  COPY . /app
  WORKDIR /app
  