How to check compressed size of image from insecure registry before docker pull?

Question

There are ways to check compressed image size before pulling from Docker Hub, but in order to do the same for an insecure/domain registry, how can I check the download size before pull?

While doing a pull, I can see size of each layer being downloaded like this, which I can sum up to find total size.

7595c8c21622: Already exists
d13af8ca898f: Already exists
70799171ddba: Already exists
b6c12202c5ef: Already exists
ef50ae158fa8: Downloading [===============>                                   ]  74.18MB/239.3MB
e5665f29b73a: Downloading [=>                                                 ]  95.26MB/2.741GB
780c121f1da3: Downloading [========>                                          ]  75.27MB/462.5MB
5fc49bebd483: Waiting
592ff9f385a7: Waiting
80a3934684b4: Waiting
b523362dbfaa: Waiting
3bf96686ba27: Waiting
dd64f3b98c5f: Waiting
937238fb3569: Waiting
c39efc826c40: Waiting

But I would like to know beforehand what would be the total download size.

Answer 1

Docker pushes a manifest file for each image on the registry, containing checksum, size and misc info, which all download clients use, in order to pull layers.

Given your registry/image name, you can inspect the manifest file, sum up the sizes of each layer to get the total download size.

First, enable Docker's experimental CLI features.

1. To temporarily enable:

   export DOCKER_CLI_EXPERIMENTAL=enabled

2. Make it permanent:

   Add "experimental": "enabled" in ~/.docker/config.json

Check size:

docker manifest inspect --insecure -v <registry_or_domain>/<image_name> | grep size | awk -F ':' '{sum+=$NF} END {print sum}' | xargs printf "%f\n" | numfmt --to=iec

printf formats the scientific notation of size (like 3e+09) to a float, which numfmt can parse.