Update and rename readme.md to readme.txt

Author: lerndevops

docker/images/readme.md renamed to docker/images/readme.txt

@@ -109,10 +109,10 @@ how to push images offline ( docker save & docker load )
 Advanced image concepts
 =======================
    find dangling images
-      docker images -f dangling=true
-      docker image prune --dangling=true
+       docker images -f dangling=true
+       docker image prune --dangling=true
 
    Inspect image metadata:
-      docker image inspect nginx:1.14.0
-      docker image inspect nginx:1.14.0 --format "{{.Architecture}}"
-      docker image inspect nginx:1.14.0 --format "{{.Architecture}} {{.Os}}"
+       docker image inspect nginx:1.14.0
+       docker image inspect nginx:1.14.0 --format "{{.Architecture}}"
+       docker image inspect nginx:1.14.0 --format "{{.Architecture}} {{.Os}}"