Docker build

External

• https://docs.docker.com/engine/reference/commandline/build/

Internal

• Image Operations

Overview

docker build expects a Dockerfile and a context and uses the information provided to produce a Docker image.

docker build [options] <path>|<url>|-

The default location for the Dockerfile is the current directory, and the path is "."

The image such produced is placed in the local registry, using the repository name and the image tag provided with -t command line option. Note that if no -t option is use, the image will still be built, but it will be "dangling". For more details see -t command line option.

The Build Process

As the Dockerfile instructions are evaluated in order, Docker inspects the build cache, looking for cached images that can be reused, instead of building a duplicate image from scratch. In matching commands to cached images, the following rules are followed:

• Starting with a parent image that is already in the cache, the next instruction is compared against all cached child images derived from that parent image, to see if one of them was built using the exact same instruction. If no such child image exists, the cache is invalidated.
• The literal instruction is used in comparison (presumably the cache maintains those instructions), with the exception of ADD and COPY.
• In case of ADD, COPY, the contents of the files to be copied are examined and a checksum is calculated for each file. The last-modified and last-accessed times of the file(s) are not considered in these checksums. The corresponding files in cache are also checksummed and the checksums are compared. If anything changed in the file - content or metadata - the cache is invalidated.
• RUN yum -y update commands require --no-cache=true to be effective.
• Once the cache is invalidated, all subsequent Dockerfile commands generate new images and the cache is not used.

The build cache can be explicitly invalidated with:

--no-cache=true

The Build Context

The build context is a set of file located in a path, or at an URL, which are specified in the build command line. The commands in the Dockerfile are relative to the context.

The URL may refer to Git repositories, pre-package TAR files and plain text files.

.dockerignore

.dockerignore

Options

-t, --tag

Even if the option is named "tag", it actually specifies the target image repository URL, ignoring the registry host name - the image is always placed in the local registry. In one of its simplest variants, the URL can be just a tag, indeed. Most commonly, the URL contains the namespace and the repository name, or just the repository name. For more details about a repository URL, see: URL.

docker build -t novaordis/centos-loop:latest .

If no name/tag information is provided, there is no default: the image may be stored with no repository and no tag, just with an image ID. An "unnamed" images is called "dangling":

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
<none>              <none>              4cfda3233bb6        5 seconds ago       204MB

Alternatively, if the only Dockerfile instruction is FROM, or ..., the name and the tag will be inferred from the base image.

If the name/tag combination already exists in the repository, the image they designate will be "unnamed" (dangled) and the new image that has just been built replaces it.

--rm

--rm=true

Remove intermediate containers after a successful build (default true).

Multi-Stage Build

https://docs.docker.com/engine/userguide/eng-image/multistage-build/

https://blog.alexellis.io/mutli-stage-docker-builds/

A more efficient replacement for the builder pattern.

Image Recipes

• Building a Container that Loops