Tutorial by Examples

A minimal Dockerfile looks like this: FROM alpine CMD ["echo", "Hello StackOverflow!"] This will instruct Docker to build an image based on Alpine (FROM), a minimal distribution for containers, and to run a specific command (CMD) when executing the resulting image. Build an...

To copy files from the build context in a Docker image, use the COPY instruction: COPY localfile.txt containerfile.txt If the filename contains spaces, use the alternate syntax: COPY ["local file", "container file"] The COPY command supports wildcards. It can be used for ...

To declare exposed ports from a Dockerfile use the EXPOSE instruction: EXPOSE 8080 8082 Exposed ports setting can be overridden from the Docker commandline but it is a good practice to explicitly set them in the Dockerfile as it helps understand what an application does.

Group common operations Docker builds images as a collection of layers. Each layer can only add data, even if this data says that a file has been deleted. Every instruction creates a new layer. For example: RUN apt-get -qq update RUN apt-get -qq install some-package Has a couple of downsides: ...

USER daemon The USER instruction sets the user name or UID to use when running the image and for any RUN, CMD and ENTRYPOINT instructions that follow it in the Dockerfile.

WORKDIR /path/to/workdir The WORKDIR instruction sets the working directory for any RUN, CMD, ENTRYPOINT, COPY and ADD instructions that follow it in the Dockerfile. If the WORKDIR doesn’t exist, it will be created even if it’s not used in any subsequent Dockerfile instruction. It can be used mu...

VOLUME ["/data"] The VOLUME instruction creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers. The value can be a JSON array, VOLUME ["/var/log/"], or a plain string with multiple arguments, such a...

COPY has two forms: COPY <src>... <dest> COPY ["<src>",... "<dest>"] (this form is required for paths containing whitespace) The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the ...

ENV ENV <key> <value> ENV <key>=<value> ... The ENV instruction sets the environment variable <key> 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...

EXPOSE <port> [<port>...] The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. EXPOSE does NOT make the ports of the container accessible to the host. To do that, you must use either the -p flag to publish a range of ports or the ...

LABEL <key>=<value> <key>=<value> <key>=<value> ... The LABEL instruction adds metadata to an image. A LABEL is a key-value pair. To include spaces within a LABEL value, use quotes and backslashes as you would in command-line parsing. A few usage examples: LAB...

The CMD instruction has three forms: CMD ["executable","param1","param2"] (exec form, this is the preferred form) CMD ["param1","param2"] (as default parameters to ENTRYPOINT) CMD command param1 param2 (shell form) There can only be one CMD ins...

MAINTAINER <name> The MAINTAINER instruction allows you to set the Author field of the generated images. DO NOT USE THE MAINTAINER DIRECTIVE According to Official Docker Documentation the MAINTAINER instruction is deprecated. Instead, one should use the LABEL instruction to define the aut...

FROM <image> Or FROM <image>:<tag> Or FROM <image>@<digest> 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 ...

RUN has 2 forms: RUN <command> (shell form, the command is run in a shell, which by default is /bin/sh -c on Linux or cmd /S /C on Windows) RUN ["executable", "param1", "param2"] (exec form) The RUN instruction will execute any commands in a new layer on top ...

ONBUILD [INSTRUCTION] The ONBUILD instruction adds to the image a trigger instruction to be executed at a later time, when the image is used as the base for another build. The trigger will be executed in the context of the downstream build, as if it had been inserted immediately after the FROM in...

STOPSIGNAL signal The STOPSIGNAL instruction sets the system call signal that will be sent to the container to exit. This signal can be a valid unsigned number that matches a position in the kernel’s syscall table, for instance 9, or a signal name in the format SIGNAME, for instance SIGKILL.

The HEALTHCHECK instruction has two forms: HEALTHCHECK [OPTIONS] CMD command (check container health by running a command inside the container) HEALTHCHECK NONE (disable any healthcheck inherited from the base image) The HEALTHCHECK instruction tells Docker how to test a container to check that...

SHELL ["executable", "parameters"] The SHELL instruction allows the default shell used for the shell form of commands to be overridden. The default shell on Linux is ["/bin/sh", "-c"], and on Windows is ["cmd", "/S", "/C"]. The...

Run the install on a single run command to merge the update and install. If you add more packages later, this will run the update again and install all the packages needed. If the update is run separately, it will be cached and package installs may fail. Setting the frontend to noninteractive and pa...