Eu não acho que você esteja fazendo certo ...
[alexus@wcmisdlin02 ~]$ docker pull nginx
Using default tag: latest
Trying to pull repository docker.io/library/nginx ...
latest: Pulling from docker.io/library/nginx
683abbb4ea60: Pull complete
a470862432e2: Pull complete
977375e58a31: Pull complete
Digest: sha256:a65beb8c90a08b22a9ff6a219c2f363e16c477b6d610da28fe9cba37c2c3a2ac
Status: Downloaded newer image for docker.io/nginx:latest
[alexus@wcmisdlin02 ~]$ docker run -d nginx
45b6daf507a37a713ed9619931e85c5e31bc17093f85d43017e98545e8d80b0c
[alexus@wcmisdlin02 ~]$ docker run -d nginx
fb57c1a1b541de6e6136b912e14af86cfbefff5e2f4d347a1819e8cc5a701903
[alexus@wcmisdlin02 ~]$ docker run -d nginx
3e343317bc4f5502647c1c2d5f34d00339cf9eb82bc95b6ff40a5d543fe4210c
[alexus@wcmisdlin02 ~]$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
3e343317bc4f nginx "nginx -g 'daemon ..." 2 seconds ago Up 1 second 80/tcp youthful_jennings
fb57c1a1b541 nginx "nginx -g 'daemon ..." 4 seconds ago Up 2 seconds 80/tcp zen_cray
45b6daf507a3 nginx "nginx -g 'daemon ..." 5 seconds ago Up 3 seconds 80/tcp laughing_raman
[alexus@wcmisdlin02 ~]$