Dockerfile cheatsheet : Pradyot H Adavi

Dockerfile cheatsheet

25 Feb 2021

What is Dockerfile?

It is a text document which contains instructions that enables to assemble an image.

Using the docker build command an image is built from Dockerfile and a context. A context is the set of files specified using either PATH or URL.

$ docker build .

$ docker build -f /path/to/Dockerfile .

Each instruction is run independently, and causes a new image to be created.
Instructions are not case-sensitive. It is conventional to keep them in UPPERCASE.
Dockerfile MUST begin with a FROM instruction.

Parser directives

Once a comment, empty line or builder instruction has been processed, Docker no longer looks for parser directives.
Convention to write parse directives in lowercase and also include a blank line after each directive.

# escape=\

dockerignore file

During the COPY instruction, all files and directories mentioned in .dockerignore are excluded from copying to the context directory.
Matching is done using Go’s filepath.Match rules. A preprocessing step removes leading and trailing whitespace and eliminates . and .. elements using Go’s filepath.Clean. Lines that are blank after preprocessing are ignored.
! is used for making exceptions to the exclusions.

FROM

Each FROM instruction clears any state created by previous instruction.
ARG is the only instruction that may precede FROM in the Dockerfile.

RUN

The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the Dockerfile.
commits are cheaper
The exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

CMD

There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect.
RUN actually runs a command and commits the result; CMD does not execute anything at build time, but specifies the intended command for the image.

LABEL

Labels included in base or parent images (images in the FROM line) are inherited by your image. If a label already exists but with a different value, the most-recently-applied value overrides any previously-set value.

docker image inspect --format='' myimage

EXPOSE

The EXPOSE instruction does not actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published.
Regardless of the EXPOSE settings, you can override them at runtime by using the -p flag

ENV

ENV abc=hello
ENV abc=bye def=$abc
ENV ghi=$abc

Output is def = hello, ghi = bye
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 =.

ADD

ADD [--chown=<user>:<group>] <src>... <dest>
ADD [--chown=<user>:<group>] ["<src>",... "<dest>"]

The ADD instruction copies new files, directories or remote file URLs from and adds them to the filesystem of the image at the path .
ADD instruction is also used for unpacking .tar, .gz etc

COPY

COPY [--chown=<user>:<group>] <src>... <dest>
COPY [--chown=<user>:<group>] ["<src>",... "<dest>"]

COPY instruction cannot be used for unpacking and remote file URLs

ENTRYPOINT

VOLUME

USER

WORKDIR

The WORKDIR instruction can be used multiple times in a Dockerfile. If a relative path is provided, it will be relative to the path of the previous WORKDIR instruction.