doc(Makefile): describe some environment variables

Makefile

@@ -5,27 +5,35 @@
 UID?=$(shell id --user)
 GID?=$(shell id --group)
 
-IMAGE_VERSION:=$(or ${VERSION}, latest)
-
 # VERSION/RELEASE
 # If no version is specified as a parameter of make, the last git hash
 # value is taken.
 VERSION?=$(shell git describe --abbrev=0)+hash.$(shell git rev-parse --short HEAD)
 RELEASE?=1
 
+# CONTAINER_IMAGE_VERSION
+# Defines the version of the container image which has included the executable
+# binary. This is somethimes different to the variable VERSION, because the
+# container image version should be latest instead contains the latest git tag
+# and hash sum.
+CONTAINER_IMAGE_VERSION:=$(or ${VERSION}, latest)
 
-# CONTAINER_RUNTIME
+# CONTAINER_RUNTIME/BUILD_IMAGE
+# The CONTAINER_RUNTIME variable will be used to specified the path to a
+# container runtime. This is needed to start and run a container image defined
+# by the BUILD_IMAGE variable. The BUILD_IMAGE container serve as build
+# environment to execute the different make steps inside. Therefore, the bulid
+# environment requires all necessary dependancies to build this project.
 CONTAINER_RUNTIME?=$(shell which docker)
+BUILD_IMAGE:=volkerraschek/build-image:latest
 
 # GH_USER/GITHUB_TOKEN
 # It's the user name from github.com and his token. This token and the username
-# can be set over encrypted environment variables in the ci/cd pipeline
+# can be set over encrypted environment variables in the ci/cd pipeline. It's
+# necessary to push the releases on github.com
 GITHUB_USER=volker-raschek
 GITHUB_TOKEN?=""
 
-# BUILD_IMAGE
-BUILD_IMAGE:=volkerraschek/build-image:latest
-
 # EXECUTABLE
 # Executable binary which should be compiled for different architecures
 EXECUTABLE:=flucky