Docker

Table of Contents

1. Dockerfile: Using build arguments with FROM statement

ARG VERSION=1.0.0
FROM image:${VERSION}

docker build --build-arg="VERSION=1.2.0" .

2. Get the Id of a container image

docker image ls --no-trunc --filter 'reference=REPO:TAG' --format '{{.ID}}'

3. Deleting dangling container images

docker rmi $(docker images -f "dangling=true" -q)

And because I mainly use fish shell: 

docker rmi (docker images -f "dangling=true" -q)

4. TODO --format

5. Docker image, repository, etc. nomenclature

I keep getting confused between image tags, image name, image reference, repository, registry and etc. I figured out why.

5.1. Starting with the docker's glossary:

  • A registry is a hosted service containing repositories of images which responds to the Registry API.
  • A repository is a set of Docker images.
  • A repository can be shared by pushing it to a registry server.
  • The different images in the repository can be labeled using tags.
  • A tag is a label applied to a Docker image in a repository.
  • An image is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime. An image typically contains a union of layered filesystems stacked on top of each other.

Notice that there's no entry in the glossary for name, image name nor references. But image name is used in docker tag (docker image tag).

5.2. docker tag gives some clarity

Usage: docker image tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG]
Alias docker tag

A full image name has the following format and components:

[HOST[:PORT_NUMBER]/]PATH

SOURCE_IMAGE and TARGET_IMAGE are either image names or image Ids.

  • HOST is a standard DNS hostname, without underscores
    • docker.io is Docker's public registry.
  • PATH consists of slash-separated components.
    • OCI Distribution Specification supports more than two slash-separated components
    • most registries only support two slash-separated components (like Docker's public registry).
    • Docker's public registry's PATH format is: [NAMESPACE/]REPOSITORY
      • where NAMESPACE defaults to library

5.3. docker pull confuses things even more

docker image pull [OPTIONS] NAME[:TAG|@DIGEST] […] docker image pull debian […] So far, you've pulled images by their name (and "tag").

Technically, those were pulled by their repository, not their "name".

5.4. Concrete example

What Value
image name docker.io/library/alpine:latest
registry docker.io
namespace library
repository alpine
tag latest

And docker.io/library/alpine:latest is equivalent to docker.io/alpine:latest because Docker's public registry's default namespace is library. It is also equivalent to alpine:latest because the default registry is (usually) docker.io.

A full image name has the following format and components:

[HOST[:PORT_NUMBER]/]PATH[:TAG]

5.5. Notes on AWS ECR   aws

  • ECR stands for Elastic Container Registry
  • There's one registry per region
  • =ImageUri=s are image names
    • e.g. ACCOUNT_ID.dkr.ecr.REGION.amazonaws.com/REPOSITORY[:TAG]

Created: 2025-03-20 Thu 01:16