add ability to read values from environment variables

It makes sense to read some values from environment. In particular this
feature is useful in case of running application in containers in ci/cd

See #108 for more details

docker/entrypoint.sh

@@ -59,7 +59,7 @@ systemd-machine-id-setup &> /dev/null
 if [ -n "$AHRIMAN_FORCE_ROOT" ]; then
     AHRIMAN_EXECUTABLE=("ahriman")
 elif ahriman help-commands-unsafe -- "$@" &> /dev/null; then
-    AHRIMAN_EXECUTABLE=("sudo" "-u" "$AHRIMAN_USER" "--" "ahriman")
+    AHRIMAN_EXECUTABLE=("sudo" "-E" "-u" "$AHRIMAN_USER" "--" "ahriman")
 else
     AHRIMAN_EXECUTABLE=("ahriman")
 fi