Run your image as a container
Prerequisites
Work through the steps to build a Python image in Build your Python image.
Overview
In the previous module, you got the sample application and then created a Dockerfile that you used to produce an image. You created your image using the Docker command docker build. Now that you have an image, you can run that image and see if your application is running correctly.
A container is a normal operating system process except that this process is isolated in that it has its own file system, its own networking, and its own isolated process tree separate from the host.
To run an image inside of a container, use the docker run command. The docker run command requires one parameter which is the name of the image. Start your image and make sure it is running correctly. Run the following command in your terminal.
$ docker run python-docker
After running this command, you’ll notice that you were not returned to the command prompt. This is because your application is a REST server and runs in a loop waiting for incoming requests without returning control back to the OS until you stop the container.
Open a new terminal then make a GET request to the server using the curl command.
$ curl localhost:5000
curl: (7) Failed to connect to localhost port 5000: Connection refused
As you can see, your curl command failed because the connection to your server was refused. This means you were not able to connect to localhost on port 5000. This is expected because your container is running in isolation which includes networking. Stop the container and restart with port 5000 published on your local network.
To stop the container, press ctrl-c. This will return you to the terminal prompt.
To publish a port for your container, use the --publish flag (-p for short) on the docker run command. The format of the --publish command is [host port]:[container port]. So, if you wanted to expose port 5000 inside the container to port 3000 outside the container, you would pass 3000:5000 to the --publish flag.
You did not specify a port when running the flask application in the container and the default is 5000. If you want your previous request going to port 5000 to work, you can map the host's port 8000 to the container's port 5000:
$ docker run --publish 8000:5000 python-docker
Now, open a new terminal window and re-run the curl command.
$ curl localhost:8000
Hello, Docker!
Success! You were able to connect to the application running inside of your container on port 8000. Switch back to the terminal where your container is running and you should see the GET request logged to the console.
[31/Jan/2021 23:39:31] "GET / HTTP/1.1" 200 -Press ctrl-c to stop the container.
Run in detached mode
This is great so far, but your sample application is a web server and you don't have to be connected to the container. Docker can run your container in detached mode or in the background. To do this, you can use the --detach or -d for short. Docker starts your container the same as before but this time will “detach” from the container and return you to the terminal prompt.
$ docker run -d -p 8000:5000 python-docker
ce02b3179f0f10085db9edfccd731101868f58631bdf918ca490ff6fd223a93b
Docker started your container in the background and printed the container ID on the terminal.
Again, make sure that your container is running properly. Run the same curl command from above.
$ curl localhost:8000
Hello, Docker!
List containers
Since you ran your container in the background, how do you know if your container is running or what other containers are running on your machine? Well, to see a list of containers running on your machine, run docker ps. This is similar to how the ps command is used to see a list of processes on a Linux machine.
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
ce02b3179f0f python-docker "python3 -m flask ru…" 6 minutes ago Up 6 minutes 0.0.0.0:8000->5000/tcp wonderful_kalam
The docker ps command provides a bunch of information about your running containers. You can see the container ID, the image running inside the container, the command that was used to start the container, when it was created, the status, ports that were exposed, and the name of the container.
You are probably wondering where the name of your container is coming from. Since you didn’t provide a name for the container when you started it, Docker generated a random name. You’ll fix this in a minute, but first you need to stop the container. To stop the container, run the docker stop command which does just that, stops the container. You need to pass the name of the container or you can use the container ID.
$ docker stop wonderful_kalam
wonderful_kalam
Now, rerun the docker ps command to see a list of running containers.
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
Stop, start, and name containers
You can start, stop, and restart Docker containers. When you stop a container, it is not removed, but the status is changed to stopped and the process inside the container is stopped. When you ran the docker ps command in the previous module, the default output only shows running containers. When you pass the --all or -a for short, you see all containers on your machine, irrespective of their start or stop status.
$ docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
ce02b3179f0f python-docker "python3 -m flask ru…" 16 minutes ago Exited (0) 5 minutes ago wonderful_kalam
ec45285c456d python-docker "python3 -m flask ru…" 28 minutes ago Exited (0) 20 minutes ago agitated_moser
fb7a41809e5d python-docker "python3 -m flask ru…" 37 minutes ago Exited (0) 36 minutes ago goofy_khayyam
You should now see several containers listed. These are containers that you started and stopped but have not been removed.
Restart the container that you just stopped. To do this, locate the name of the container you just stopped and replace the name of the container below in the restart command.
$ docker restart wonderful_kalam
Now list all the containers again using the docker ps command.
$ docker ps --all
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
ce02b3179f0f python-docker "python3 -m flask ru…" 19 minutes ago Up 8 seconds 0.0.0.0:8000->5000/tcp wonderful_kalam
ec45285c456d python-docker "python3 -m flask ru…" 31 minutes ago Exited (0) 23 minutes ago agitated_moser
fb7a41809e5d python-docker "python3 -m flask ru…" 40 minutes ago Exited (0) 39 minutes ago goofy_khayyam
Notice that the container you just restarted has been started in detached mode and has port 8000 exposed. Also, observe the status of the container is “Up X seconds”. When you restart a container, it starts with the same flags or commands that it was originally started with.
Now, stop and remove all of your containers and take a look at fixing the random naming issue. Stop the container you just started. Find the name of your running container and replace the name in the command below with the name of the container on your system.
$ docker stop wonderful_kalam
wonderful_kalam
Now that all of your containers are stopped, remove them. When you remove a container, it is no longer running, nor it is in the stopped status, but the process inside the container has been stopped and the metadata for the container has been removed.
$ docker ps --all
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
ce02b3179f0f python-docker "python3 -m flask ru…" 19 minutes ago Exited (0) 5 seconds ago wonderful_kalam
ec45285c456d python-docker "python3 -m flask ru…" 31 minutes ago Exited (0) 23 minutes ago agitated_moser
fb7a41809e5d python-docker "python3 -m flask ru…" 40 minutes ago Exited (0) 39 minutes ago goofy_khayyam
To remove a container, run the docker rm command with the container name. You can pass multiple container names to the command using a single command. Again, replace the container names in the following command with the container names from your system.
$ docker rm wonderful_kalam agitated_moser goofy_khayyam
wonderful_kalam
agitated_moser
goofy_khayyam
Run the docker ps --all command again to see that all containers are removed.
Now, address the random naming issue. Standard practice is to name your containers for the simple reason that it is easier to identify what is running in the container and what application or service it is associated with.
To name a container, you just need to pass the --name flag to the docker run command.
$ docker run -d -p 8000:5000 --name rest-server python-docker
1aa5d46418a68705c81782a58456a4ccdb56a309cb5e6bd399478d01eaa5cdda
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
1aa5d46418a6 python-docker "python3 -m flask ru…" 3 seconds ago Up 3 seconds 0.0.0.0:8000->5000/tcp rest-server
That’s better! You can now easily identify your container based on the name.
Summary
In this section, you took a look at running containers. You also took a look at managing containers by starting, stopping, and restarting them. And finally, you looked at naming your containers so they are more easily identifiable.
Related information:
Next steps
In the next section, you’ll learn how to run a database in a container and connect it to a Rust application.