New explicit Docker selection based on environment variable

Everything else breaks after a few years I give up.

setup

@@ -6,14 +6,18 @@ if [ $# -eq 1 ]; then
 else
   y=
 fi
-indocker="$(set +e; grep -q docker /proc/1/cgroup; echo $?)"
-if [ "$indocker" -eq 0 ]; then
+if [ true = "$LKMC_IN_DOCKER" ]; then
+  indocker=true
+else
+  indocker=false
+fi
+if "$indocker"; then
   sudo=
   export DEBIAN_FRONTEND=noninteractive
 else
   sudo=sudo
 fi
-if [ "$indocker" -eq 0 ]; then
+if "$indocker"; then
   $sudo apt update
   apt install -y software-properties-common
   add-apt-repository -y ppa:git-core/ppa