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Split long or complex RUN statements on multiple lines separated withbackslashes to make your Dockerfile more readable, understandable, andmaintainable.
Probably the most common use-case for RUN is an application of apt-get. Because it installs packages, the RUN apt-get command has several gotchas tolook out for. For example:. Using apt-get update alone in a RUN statement causes caching issues andsubsequent apt-get install instructions fail. For example, say you have aDockerfile:. After building the image, all layers are in the Docker cache. Suppose you latermodify apt-get install by adding extra package:. Docker sees the initial and modified instructions as identical and reuses thecache from previous steps.
As a result the apt-get update is not executedbecause the build uses the cached version. Because the apt-get update is notrun, your build can potentially get an outdated version of the curl and nginx packages.
You can also achievecache-busting by specifying a package version. This is known as version pinning,for example:. This technique can also reduce failures due to unanticipated changesin required packages. Below is a well-formed RUN instruction that demonstrates all the apt-get recommendations.
The s3cmd argument specifies a version 1. If the image previouslyused an older version, specifying the new one causes a cache bust of apt-getupdate and ensures the installation of the new version.
Listing packages oneach line can also prevent mistakes in package duplication. Since the RUN statement starts with apt-get update , the package cache is alwaysrefreshed prior to apt-get install. Official Debian and Ubuntu images automatically run apt-get clean ,so explicit invocation is not required.
Some RUN commands depend on the ability to pipe the output of one command into another, using the pipe character , as in the following example:. In the example above this build step succeeds and produces a new image so longas the wc -l command succeeds, even if the wget command fails. In cases such as the dash shell onDebian-based images, consider using the exec form of RUN to explicitlychoose a shell that does support the pipefail option. The CMD instruction should be used to run the software contained in yourimage, along with any arguments.
Indeed, this form of the instruction is recommendedfor any service-based image. In most other cases, CMD should be given an interactive shell, such as bash,python and perl. Consequently, you should use the common, traditional port foryour application. For external access, your users can execute docker run with a flag indicatinghow to map the specified port to the port of their choice.
Lastly, ENV can also be used to set commonly used version numbers so thatversion bumps are easier to maintain, as seen in the following example:. Similar to having constant variables in a program as opposed to hard-codingvalues , this approach lets you change a single ENV instruction toauto-magically bump the version of the software in your container. Thismeans that even if you unset the environment variable in a future layer, itstill persists in this layer and its value can be dumped. You can test this bycreating a Dockerfile like the following, and then building it.
To prevent this, and really unset the environment variable, use a RUN commandwith shell commands, to set, use, and unset the variable all in a single layer. If you use the second method,and one of the commands fails, the docker build also fails. This is usually agood idea. Using as a line continuation character for Linux Dockerfilesimproves readability. You could also put all of the commands into a shell scriptand have the RUN command just run that shell script.
COPY onlysupports the basic copying of local files into the container, while ADD hassome features like local-only tar extraction and remote URL support that arenot immediately obvious. If you have multiple Dockerfile steps that use different files from yourcontext, COPY them individually, rather than all at once. Because image size matters, using ADD to fetch packages from remote URLs isstrongly discouraged; you should use curl or wget instead.
For example, you should avoid doingthings like:. This is useful because the image name can double as a reference to the binary asshown in the command above. Thisallows the application to receive any Unix signals sent to the container. If a service can run without privileges, use USER to change to a non-rootuser.
A workaround is to pass the --no-log-init flag touseradd. Avoid installing or using sudo as it has unpredictable TTY andsignal-forwarding behavior that can cause problems. Lastly, to reduce layers and complexity, avoid switching USER back and forthfrequently.
Adding a separate tag, as recommended above, helps mitigate this byallowing the Dockerfile author to make a choice. Skip to content Dockerfile Example. Estimated reading time: 31 minutes This document covers recommended best practices and methods for buildingefficient images. RUN builds your application with make.
CMD specifies what command to run within the container. For more on image layers and how Docker builds and stores images , seeAbout storage drivers.
General guidelines and recommendations Create ephemeral containers The image defined by your Dockerfile should generate containers that are asephemeral as possible. Understand build context When you issue a docker build command, the current working directory is calledthe build context. Build context example Create a directory for the build context and cd into it. Use -f to point to the Dockerfile and specify the directory of the build context: Inadvertently including files that are not necessary for building an imageresults in a larger build context and larger image size.
To see how big your build context is, look for a message like this whenbuilding your Dockerfile : Pipe Dockerfile through stdin Docker has the ability to build images by piping Dockerfile through stdin with a local or remote build context.
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