tests/docker: add support for DEB_KEYRING

For installing stuff from sid or ports you may need to manually
specify the location of the keyring. You can even import keys into
your personal keyring and point it there, e.g.:

  gpg --keyserver keyring.debian.org --recv-keys 84C573CD4E1AFD6C
  make docker-binfmt-image-debian-sid-hppa DEB_TYPE=sid DEB_ARCH=hppa \
      DEB_URL=http://ftp.ports.debian.org/debian-ports/ \
      EXECUTABLE=./hppa-linux-user/qemu-hppa V=1 \
      DEB_KEYRING=${HOME}/.gnupg/pubring.kbx

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200724064509.331-12-alex.bennee@linaro.org>

tests/docker/dockerfiles/debian-bootstrap.pre

@@ -79,6 +79,13 @@ else
     fi
 fi
 
+#
+# Add optional args
+#
+if [ -n "${DEB_KEYRING}" ]; then
+    DEBOOTSTRAP="${DEBOOTSTRAP} --keyring=${DEB_KEYRING}"
+fi
+
 #
 # Finally check to see if any qemu's are installed
 #