configure: add test for docker availability

This tests for a working docker installation without sudo and sets up
config-host.mak accordingly. This will be useful from cross compiling
things in the future.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>

configure

@@ -456,6 +456,7 @@ jemalloc="no"
 replication="yes"
 vxhs=""
 libxml2=""
+docker="no"
 
 supported_cpu="no"
 supported_os="no"
@@ -5450,6 +5451,17 @@ EOF
   fi
 fi
 
+##########################################
+# Docker and cross-compiler support
+#
+# This is specifically for building test
+# cases for foreign architectures, not
+# cross-compiling QEMU itself.
+
+if has "docker"; then
+    docker=$($python $source_path/tests/docker/docker.py probe)
+fi
+
 ##########################################
 # End of CC checks
 # After here, no more $cc or $ld runs
@@ -5913,6 +5925,7 @@ echo "avx2 optimization $avx2_opt"
 echo "replication support $replication"
 echo "VxHS block device $vxhs"
 echo "capstone          $capstone"
+echo "docker            $docker"
 
 if test "$sdl_too_old" = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
@@ -6740,6 +6753,10 @@ if test "$gcov" = "yes" ; then
   echo "GCOV=$gcov_tool" >> $config_host_mak
 fi
 
+if test "$docker" != "no"; then
+    echo "HAVE_USER_DOCKER=y" >> $config_host_mak
+fi
+
 # use included Linux headers
 if test "$linux" = "yes" ; then
   mkdir -p linux-headers