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Fedora - Building Smaller Container Images - xSicKxBot - 05-17-2019
Building Smaller Container Images
<div><p>Linux Containers have become a popular topic, making sure that a container image is not bigger than it should be is considered as a good practice. This article give some tips on how to create smaller Fedora container images.</p>
<p> <span id="more-27793"></span> </p>
<h3>microdnf</h3>
<p>Fedora’s DNF is written in Python and and it’s designed to be extensible as it has wide range of plugins. But Fedora has an alternative base container image which uses an smaller package manager called <a href="https://github.com/rpm-software-management/microdnf">microdnf</a> written in C. To use this minimal image in a Dockerfile the FROM line should look like this:</p>
<pre class="wp-block-preformatted">FROM registry.fedoraproject.org/fedora-minimal:30</pre>
<p>This is an important saving if your image does not need typical DNF dependencies like Python. For example, if you are making a NodeJS image.</p>
<h3>Install and Clean up in one layer</h3>
<p>To save space it’s important to remove repos meta data using <em>dnf clean all</em> or its microdnf equivalent <em>microdnf clean all</em>. But you should not do this in two steps because that would actually store those files in a container image layer then mark them for deletion in another layer. To do it properly you should do the installation and cleanup in one step like this</p>
<pre class="wp-block-preformatted">FROM registry.fedoraproject.org/fedora-minimal:30 <br />RUN microdnf install nodejs && microdnf clean all</pre>
<h3>Modularity with microdnf</h3>
<p>Modularity is a way to offer you different versions of a stack to choose from. For example you might want non-LTS NodeJS version 11 for a project and old LTS NodeJS version 8 for another and latest LTS NodeJS version 10 for another. You can specify which stream using colon</p>
<pre class="wp-block-preformatted"># dnf module list <br /># dnf module install nodejs:8</pre>
<p>The <em>dnf module install</em> command implies two commands one that enables the stream and one that install nodejs from it.</p>
<pre class="wp-block-preformatted"># dnf module enable nodejs:8 <br /># dnf install nodejs</pre>
<p>Although microdnf does not offer any command related to modularity, it is possible to enable a module with a configuation file, and libdnf (which microdnf uses) <a href="https://bugzilla.redhat.com/show_bug.cgi?id=1575626">seems</a> to support modularity streams. The file looks like this</p>
<pre class="wp-block-preformatted">/etc/dnf/modules.d/nodejs.module <br />[nodejs] <br />name=nodejs <br />stream=8 <br />profiles= <br />state=enabled</pre>
<p>A full Dockerfile using modularity with microdnf looks like this: </p>
<pre class="wp-block-preformatted">FROM registry.fedoraproject.org/fedora-minimal:30 <br />RUN \<br /> echo -e "[nodejs]\nname=nodejs\nstream=8\nprofiles=\nstate=enabled\n" > /etc/dnf/modules.d/nodejs.module && \<br /> microdnf install nodejs zopfli findutils busybox && \<br /> microdnf clean all</pre>
<h3>Multi-staged builds</h3>
<p>In many cases you might have tons of build-time dependencies that are not needed to run the software for example building a Go binary, which statically link dependencies. Multi-stage build are an efficient way to separate the application build and the application runtime.</p>
<p>For example the Dockerfile below builds <a href="https://github.com/kelseyhightower/confd">confd</a> a Go application.</p>
<pre class="wp-block-preformatted"># building container <br />FROM registry.fedoraproject.org/fedora-minimal AS build <br />RUN mkdir /go && microdnf install golang && microdnf clean all <br />WORKDIR /go <br />RUN export GOPATH=/go; CGO_ENABLED=0 go get github.com/kelseyhightower/confd <br /><br />FROM registry.fedoraproject.org/fedora-minimal <br />WORKDIR / <br />COPY --from=build /go/bin/confd /usr/local/bin <br />CMD ["confd"] </pre>
<p>The multi-stage build is done by adding <em>AS</em> after the <em>FROM</em> instruction and by having another <em>FROM</em> from a base container image then using C<em>OPY –from=</em> instruction to copy content from the <em>build</em> container to the second container.</p>
<p>This Dockerfile can then be built and run using podman</p>
<pre class="wp-block-preformatted">$ podman build -t myconfd .<br />$ podman run -it myconfd</pre>
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