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Dockerfile Instructions

  • The instructions in a Dockerfile are executed in order from top to bottom. Each instruction creates a new layer on top of the previous one.
  • The layers are committed when instructions like CMD, COPY, ADD, etc. are encountered.
  • The FROM instruction initializes the base image and should be the first instruction in any Dockerfile.
  • The order of instructions matters and determines the step-by-step build process of the image.

Popular Dockerfile Instructions

  • FROM: Sets the base image.
  • MAINTAINER: Author/Maintainer information.
  • ENV: Sets environment variable.
  • ADD: Copies files to the image.
  • COPY: Copies files or directories to the image
  • RUN: Execute commands in a new layer
  • ENTRYPOINT: Sets the primary command to be executed at container startup
  • CMD: Default command to execute at container startup
  • USER: Sets the user to use when running the image
  • WORKDIR: Sets the working directory for future commands
  • ARG: Allows ARG instructions to define build-time variables
  • ONBUILD: Adds a trigger instruction to be executed when the image is built
  • STOPSIGNAL: Sets the default signal to stop a container
  • HEALTHCHECK: Set health check instructions
  • EXPOSE: Exposes a port for users of the image
  • VOLUME: Creates a mount point within the image

Understanding each Dockerfile Instruction

  1. FROM instruction The FROM instruction sets the Base Image for subsequent instructions. As such, a valid Dockerfile must have FROM as its first instruction. The image can be any valid image. It is especially easy to start by pulling an image from the Public Repositories.

  • FROM must be the first non-comment instruction in the Dockerfile.

  • FROM can appear multiple times within a single Dockerfile in order to create multiple images

  • Usage:

FROM <image>

Or

FROM <image>:<tag>

Or

FROM <image>@<digest>

The tag or digest values are optional. If you omit either of them, the builder assumes a latest by default. The builder returns an error if it cannot match the tag value.

  1. MAINTAINER Instruction The MAINTAINER instruction allows you to set the Author field of the generated images.
MAINTAINER <name>
  1. ENV Instruction The ENV instruction sets the environment variable to the value. This value will be in the environment of all “descendant” Dockerfile commands and can be replaced inline in many as well. The ENV instruction has two forms.
ENV <key> <value> 

ENV <key>=<value> ...
  • The first form, ENV , will set a single variable to a value. The entire string after the first space will be treated as the - including characters such as spaces and quotes.
  • The second form, ENV = ..., allows for multiple variables to be set at one time. Notice that the second form uses the equals sign (=) in the syntax, while the first form does not. The environment variables set using ENV will persist when a container is run from the resulting image.

You can view the values using docker inspect, and change them using docker run --env =.

  1. ADD Instruction The ADD instruction copies files or directories from one location to another. When using Docker, there are a couple of use cases for ADD:
  • Copying files from your host machine to the image:
ADD hello.txt /some-dir/

This will copy hello.txt from your current directory on the host into some-dir in the image.

  • Copying directories recursively from your host machine to the image:
ADD . /app/

This will copy the entire current directory into /app in the image.

  1. COPY Instruction The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>.

The copy command has two forms,

COPY <src>... <dest> 

COPY ["<src>",... "<dest>"] // (this form is required for paths containing whitespace)

Multiple <src> resource may be specified but they must be relative to the source directory that is being built (the context of the build).

Each <src> may contain wildcards. For example

COPY hom* /mydir/        # adds all files starting with "hom" 

COPY hom?.txt /mydir/    # ? is replaced with any single character, e.g., "home.txt"

The <dest> is an absolute path, or a path relative to WORKDIR, into which the source will be copied inside the destination container.

COPY test relativeDir/   # adds "test" to `WORKDIR`/relativeDir/ 

COPY test /absoluteDir/  # adds "test" to /absoluteDir/