Building Smaller Container Images

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.

microdnf

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 microdnf written in C. To use this minimal image in a Dockerfile the FROM line should look like this:

FROM registry.fedoraproject.org/fedora-minimal:30

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.

Install and Clean up in one layer

To save space it’s important to remove repos meta data using dnf clean all or its microdnf equivalent microdnf clean all. 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

FROM registry.fedoraproject.org/fedora-minimal:30 
RUN microdnf install nodejs && microdnf clean all

Modularity with microdnf

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

# dnf module list  
# dnf module install nodejs:8

The dnf module install command implies two commands one that enables the stream and one that install nodejs from it.

# dnf module enable nodejs:8 
# dnf install nodejs

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) seems to support modularity streams. The file looks like this

/etc/dnf/modules.d/nodejs.module 
[nodejs] 
name=nodejs 
stream=8 
profiles= 
state=enabled

A full Dockerfile using modularity with microdnf looks like this:

FROM registry.fedoraproject.org/fedora-minimal:30 
RUN \
   echo -e "[nodejs]\nname=nodejs\nstream=8\nprofiles=\nstate=enabled\n" > /etc/dnf/modules.d/nodejs.module && \
   microdnf install nodejs zopfli findutils busybox && \
   microdnf clean all

Multi-staged builds

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.

For example the Dockerfile below builds confd a Go application.

# building container 
FROM registry.fedoraproject.org/fedora-minimal AS build 
RUN mkdir /go && microdnf install golang && microdnf clean all 
WORKDIR /go 
RUN export GOPATH=/go; CGO_ENABLED=0 go get github.com/kelseyhightower/confd 

FROM registry.fedoraproject.org/fedora-minimal 
WORKDIR / 
COPY --from=build /go/bin/confd /usr/local/bin 
CMD ["confd"] 

The multi-stage build is done by adding AS after the FROM instruction and by having another FROM from a base container image then using COPY –from= instruction to copy content from the build container to the second container.

This Dockerfile can then be built and run using podman

$ podman build -t myconfd .
$ podman run -it myconfd