Docker portability issues with locally saved images

Question

I am currently working on a project that requires an airtight cloud infrastructure. Thus all artefacts, including docker images, have to be available offline.

It appears that the docker save and the subsequent docker import do not result in an equal Docker image.

Docker version: 1.10.3

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED            SIZE

$ docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

$ docker pull registry:2
2: Pulling from library/registry
fdd5d7827f33: Pull complete 
a3ed95caeb02: Pull complete 
a79b4a92697e: Pull complete 
6cbb75c7cc30: Pull complete 
4831699594bc: Pull complete 
Digest:    sha256:20f5d95004b71fe14dbe7468eff33f18ee7fa52502423c5d107d4fb0abb05c1d
Status: Downloaded newer image for registry:2

$ docker run -p 5000:5000 -v /data/docker_images:/tmp/registry-dev registry:2 
time="2016-04-01T19:11:55Z" level=warning msg="No HTTP secret provided - generated random secret. This may cause problems with uploads if multiple registries are behind a load-balancer. To provide a shared secret, fill in http.secret in the configuration file or set the REGISTRY_HTTP_SECRET environment variable." go.version=go1.5.3 instance.id=af87e7b7-9b56-49a6-bf71-4bd04e141c95 version=v2.3.1 
time="2016-04-01T19:11:55Z" level=info msg="redis not configured" go.version=go1.5.3 instance.id=af87e7b7-9b56-49a6-bf71-4bd04e141c95 version=v2.3.1 
time="2016-04-01T19:11:55Z" level=info msg="using inmemory blob descriptor cache" go.version=go1.5.3 instance.id=af87e7b7-9b56-49a6-bf71-4bd04e141c95 version=v2.3.1 
time="2016-04-01T19:11:55Z" level=info msg="listening on [::]:5000" go.version=go1.5.3 instance.id=af87e7b7-9b56-49a6-bf71-4bd04e141c95 version=v2.3.1 
time="2016-04-01T19:11:55Z" level=info msg="Starting upload purge in 45m0s" go.version=go1.5.3 instance.id=af87e7b7-9b56-49a6-bf71-4bd04e141c95 version=v2.3.1

The above run successfully!

My aim is to save the Docker Registry image as a *.tar file, move it to another server, import the image there and will be able to run the Docker Registry as I could when it was pulled from Internet/ Docker Hub. Below is the sequence of commands, all run one the same machine, that led to the error.

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
registry            2                   83139345d017        3 weeks ago         165.8 MB

$ docker save 83139345d017 > registry.tar

$ docker import registry.tar foo/registry
sha256:8e84e7d07f97825b0ebaa04054f988566bb7b90083ba80caec6398e085adce84

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
foo/registry        latest              8e84e7d07f97        3 seconds ago       172.3 MB
registry            2                   83139345d017        3 weeks ago         165.8 MB

$ docker run -p 5000:5000 -v /data/tmp:/tmp/registry-dev foo/registry
docker: Error response from daemon: No command specified.
See 'docker run --help'.

Error... It somehow expects a command, whereas when pulled directly from the Docker Hub, it doesn't require a command. Also note that the image sizes are a couple of MBs different.

Is this a bug or do I miss something here?

Answer 1

@larsks comment resolved it. The problem was that I use the wrong function to import the image. The correct function to use is docker load !

Below is a sequence of commands that are suitable for creating, moving and loading Docker images.

Assumptions:

• Local Docker registry runs on IP 10.10.10.10:5000
• Docker image from Docker Hub is called stack/overflow (This image does not exist! The image name is used for illustration only.)

Here we go...

# Pull the image from public Docker Hub
$ docker pull stack/overflow:0.9

# List the images
$ docker images
stack/overflow              0.9              57cbfaed16bf        7 months ago        20.96 MB

# Change the name of the image to match your local Docker Registry address
$ docker tag 57cbfaed16bf 10.10.10.10:5000/overflow:0.9

# Save Docker image
# IMPORTANT: You need to use the "repo/tag" to select the image to be saved, as it otherwise does not save the "repo/tag" information in the tar file!
$ docker save -o overflow.tar 10.10.10.10:5000/overflow:0.9

# Copy the overflow.tar file to the offline environment, which as the Docker Registry running on 10.10.10.10:5000

# Load Docker image
$ docker load -i overflow.tar

# List the images
$ docker images
10.10.10.10:5000/overflow              0.9              38ea67f2e6d8        7 months ago        20.96 MB

# Push the image to the local registry
$ docker push 10.10.10.10:5000/overflow