Makefile.docker: Detect if running inside a Docker container

Makefile.docker

@@ -8,6 +8,14 @@ export DOCKER_MAKECMDGOALS_POSSIBLE = \
   #
 export DOCKER_MAKECMDGOALS = $(filter $(MAKECMDGOALS),$(DOCKER_MAKECMDGOALS_POSSIBLE))
 
+# Docker creates the files .dockerinit and .dockerenv in the root directory of
+# the container, we check for the files to determine if we are inside a container.
+ifneq (,$(wildcard /.dockerinit /.dockerenv))
+  export INSIDE_DOCKER := 1
+else
+  export INSIDE_DOCKER := 0
+endif
+
 # Default target for building inside a Docker container if nothing was given
 export DOCKER_MAKECMDGOALS ?= all
 # List of all exported environment variables that shall be passed on to the