Will Docker Container Restart Pick Up Updated Image?

When a Docker image has been updated, will restarting the running container via docker restart pick up the change? Educated guess will be no, because like restarting a process, the memory is still retained. The best way to find out is to give a try.

Let’s start with a Dockerfile:

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# Version Foo
FROM debian:8.5
CMD while true; do echo foo; sleep 5; done

The command will keep printing foo every 5 seconds.

Create the image:

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$ docker build -t example .
Sending build context to Docker daemon 2.048 kB
Step 1 : FROM debian:8.5
 ---> 1b088884749b
Step 2 : CMD while true; do echo foo; sleep 5; done
 ---> Running in 38fdeb15f629
 ---> 6a56a50ef254
Removing intermediate container 38fdeb15f629
Successfully built 6a56a50ef254

Notice the image ID starting with 6a56.

Start the container:

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$ docker run -d --name example example
dac42e7194e4ec2bdca8e24db29a3333ae2f422d316e341c5cb1499034a4357b

Check the log:

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$ docker logs example
foo
foo

This is expected output.

Inspect the container:

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$ docker inspect example

The important field is the corresponding image, which matches to the previous built image:

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{
  ...
  "Image": "sha256:6a56a50ef254bb1d07117b0a0750ef81fafe9735ab3b0f2b0a14511f38d5b83d"
  ...
}

Now update the Dockerfile:

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# Version Bar
FROM debian:8.5
CMD while true; do echo bar; sleep 5; done

This time it prints bar instead of foo.

Rebuild the image:

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$ docker build -t example .
Sending build context to Docker daemon 2.048 kB
Step 1 : FROM debian:8.5
 ---> 1b088884749b
Step 2 : CMD while true; do echo bar; sleep 5; done
 ---> Running in 7fc297e12005
 ---> a6c04345afb9
Removing intermediate container 7fc297e12005
Successfully built a6c04345afb9

Now we have a different image. The image ID is different: a6c0. But the old image is still there:

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$ docker images
REPOSITORY                        TAG                 IMAGE ID            CREATED             SIZE
example                           latest              a6c04345afb9        24 seconds ago      125.1 MB
<none>                            <none>              6a56a50ef254        3 minutes ago       125.1 MB

Restart the container:

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$ docker restart example
example

Got bar? No still foo all the way with the log. And when you inspect the container, it still uses the old image.

So, docker restart will not pick up the changes from updated image, it will still use the old image built previously. Therefore, the correct way is to drop the container entirely and run it again:

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$ docker stop example && docker rm example && docker run -d --name example example
example
example
55cec9110fed0257060673a085a08f143003336b1720894f43c6ac5a22104935

The log shows the correct message:

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$ docker logs example
bar
bar

Inspecting the container, now it has the correct image:

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$ docker inspect example
{
  ...
  "Image": "sha256:a6c04345afb953ab392241f56c04f72110c772a6ee3a36e248c1ffd03f81b7d6"
  ...
}

And don’t forget to delete the old image.

Settings:

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$ docker --version
Docker version 1.12.0, build 8eab29e