How can I backup this image
Simply use the docker save command. $ docker save myusername/myproject:latest | gzip -c > myproject_img_bak20141103.tgz
You will later be able to restore it with the docker load command. gunzip -c myproject_img_bak20141103.tgz | docker load
or change it to a different tag locally without committing to networking repository?
Use the docker tag command: $ docker tag myusername/myproject:latest myusername/myproject:bak20141103