Add kernel version check (#673) [ci skip]

On Ruby 3.1 there is a `RuntimeException` when calling `Random.urandom` on platforms where it's not properly implemented, which includes people running Ubuntu 14.04 with kernel 3.13.

See

https://meta.discourse.org/t/docker-build-broken-site-down/252047
https://meta.discourse.org/t/undefined-method-deprecated-for-nil-nilclass-on-rebuild/251711
https://meta.discourse.org/t/discourse-update-error/253197
https://meta.discourse.org/t/bootstrap-failure-during-most-recent-upgrade/253424

launcher

@@ -87,6 +87,7 @@ docker_min_version='17.03.1'
 docker_rec_version='17.06.2'
 git_min_version='1.8.0'
 git_rec_version='1.8.0'
+kernel_min_version='4.14.0'
 
 config_file=containers/"$config".yml
 cidbootstrap=cids/"$config"_bootstrap.cid
@@ -259,6 +260,15 @@ check_prereqs() {
     echo "WARNING: Git version ${test} deprecated, recommend upgrade to ${git_rec_version} or newer."
   fi
 
+  # Check minimum kernel version due to https://bugs.ruby-lang.org/issues/13885
+  test=($(uname -r))
+
+  # At least minimum version
+  if compare_version "${kernel_min_version}" "${test}"; then
+    echo "ERROR: Kernel version ${test} not supported, please upgrade to at least ${kernel_min_version}"
+    exit 1
+  fi
+
   # 6. able to attach stderr / out / tty
   test=`$docker_path run $user_args -i --rm -a stdout -a stderr $image echo working`
   if [[ "$test" =~ "working" ]] ; then : ; else