Dockerfile

External

• https://docs.docker.com/engine/reference/builder/

Internal

• Docker Concepts

Overview

A Dockerfile is a plain text file that defines how a container should look at build time. It contains all the steps that are required to create the image.

Each line in the Dockerfile generates a new layer in the image. Multiple commands can be combined on a single line, to reduce the number of layers.

The Dockerfile is used as the argument of the docker build command.

Syntax

<instruction> <arguments>

Example

Dockerfile Example

Instructions

Instructions are also known as "directives", and they are part of a DSL.

ADD

Copies files from the local filesystem into the image.

ADD /something/something_else.conf $MY_PATH

FROM

Specifies the base image.

FROM node:0.10

USER

The user to run a container's processes as.

Also see Docker Security

ENV

Declares environment variable accessible to the processes in the container:

ENV SOMETHING "something else"

RUN

Running commands like yum in the Dockerfile is discouraged because it increases the time it takes for the build to finish. The alternative is to use base images that already have these updates applied.

WORKDIR

Changes the working directory within the context of the image being built, for the rest of the build instructions.

CMD

Defines the command that launches the process to be executed in the container.

CMD [ "somehting", "-arg" ]

LABEL

Applies a label to the image.

LABEL "something"="something else" "other label"="some other content"

VOLUME

VOLUME /some/dir

Define a mount point to an external volume on the native host or other containers. The actual location of the volume on the native host is a directory whose path is returned by the corresponding "Source" entry in output of the "inspect -f '{{json .Mounts}}' <container-id>" command.