Docker creates a container from a existing image and runs it, the first process started by docker will have pid of 1 and if no command is specified when running container it will run the container and immediately exit.
lets see this in action
$docker images REPOSITORY TAG IMAGE ID CREATED SIZE centos latest 2d194b392dd1 3 weeks ago 195MB
starting a docker container from the existing image centos7
$docker run centos
The above command starts the docker container since there was no command passed docker container starts and exists.
Docker process can be checked from command docker ps
$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
lets run docker with shell access
$docker run centos /bin/bash
starts a container since we have not specified a tty access docker exists process.
running docker with a terminal tty access
ocker run -it centos /bin/bash [root@64e34e41902b /]#
when we exit attached docker container it is removed because it exists the process 1 (/bin/bash)
$[root@c955676bdc94 /]#
[root@c955676bdc94 /]# ps
PID TTY TIME CMD
1 pts/0 00:00:00 bash
13 pts/0 00:00:00 ps
[root@c955676bdc94 /]# exit exit
If we want to avoid this and keep container active then we need run docker in daemon mode it can be done by using -d option
$docker run -itd centos /bin/bash
67d7614a7d3b8e92d1a48ae4fdccfe330357b5ac4014b25e48ebe8fcd5f6c2fa
docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
67d7614a7d3b centos "/bin/bash" 15 seconds ago Up 14 seconds compassionate_shannon
$docker attach 67d7614a7d3b
[root@67d7614a7d3b /]#
$[root@67d7614a7d3b /]# ps
PID TTY TIME CMD
1 pts/0 00:00:00 bash
16 pts/0 00:00:00 ps