Part 3

Optimizing the image size

The bigger your image is the larger the surface area for an attack is. The following tutorial to "Building Small Containers" from Google is an excellent video to showcase the importance of optimizing your Dockerfiles:

Let's start by reducing the number of layers. To keep track of the improvements, we will follow the image size after each new Dockerfile.

FROM ubuntu:18.04

WORKDIR /usr/videos


RUN apt-get update
RUN apt-get install -y curl python
RUN curl -L -o /usr/local/bin/youtube-dl
RUN chmod a+x /usr/local/bin/youtube-dl
RUN useradd -m appuser

USER appuser

ENTRYPOINT ["/usr/local/bin/youtube-dl"]


We will glue all RUN commands together to reduce the number of layers we are making in our image.

FROM ubuntu:18.04

WORKDIR /usr/videos


RUN apt-get update && apt-get install -y \
    curl python && \
    curl -L -o /usr/local/bin/youtube-dl && \
    chmod a+x /usr/local/bin/youtube-dl && \
    useradd -m appuser

USER appuser

ENTRYPOINT ["/usr/local/bin/youtube-dl"]


As a sidenote not directly related to docker: remember that if needed, it is possible to bind packages to versions with curl=1.2.3 - this will ensure that if the image is built at the later date the image is more likely to work as the versions are exact. On the other hand, the packages will be old and have security issues.

With docker image history we can see that our single RUN layer adds 76.7 megabytes to the image:

$ docker image history youtube-dl

  IMAGE          CREATED              CREATED BY                                      SIZE      COMMENT
  f221975422c3   About a minute ago   /bin/sh -c #(nop)  ENTRYPOINT ["/usr/local/b…   0B
  940a7510dc5d   About a minute ago   /bin/sh -c #(nop)  USER appuser                 0B
  31062eddb851   About a minute ago   /bin/sh -c apt-get update && apt-get install…   76.7MB

The next step is to remove everything that is not needed in the final image. We don't need the apt source lists anymore, so we can glue the next line to our single RUN

.. && \
rm -rf /var/lib/apt/lists/*

Now, after we build, the size of the layer is 45.6MB megabytes. We can optimize even further by removing the curl. We can remove curl and all the dependencies it installed with

.. && \
apt-get purge -y --auto-remove curl && \
rm -rf /var/lib/apt/lists/*

..which brings us down to 34.9MB.

Now our slimmed down container should work, but:

$ docker container run -v "$(pwd):/usr/videos" youtube-dl

  [Imgur] JY5tHqr: Downloading webpage

  ERROR: Unable to download webpage: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (caused by URLError(SSLError(1, u'[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)'),))

Because --auto-remove also removed dependencies, like:

  Removing ca-certificates (20170717~18.04.1) ...

We can now see that our youtube-dl worked previously because of our curl dependencies. If youtube-dl would have been installed as a package, it would have declared ca-certificates as its dependency.

Now what we could do is to first purge --auto-remove and then add ca-certificates back with apt-get install or just install ca-certificates along with other packages before removing curl:

FROM ubuntu:18.04

WORKDIR /usr/videos


RUN apt-get update && apt-get install -y \
    curl python ca-certificates && \
    curl -L -o /usr/local/bin/youtube-dl && \
    chmod a+x /usr/local/bin/youtube-dl && \
    apt-get purge -y --auto-remove curl && \
    rm -rf /var/lib/apt/lists/* && \
    useradd -m appuser

USER appuser

ENTRYPOINT ["/usr/local/bin/youtube-dl"]


From the build output we can see that ca-certificates also adds openssl

  The following additional packages will be installed:

  The following NEW packages will be installed:
  ca-certificates openssl

and this brings us to 36.9 megabytes in our RUN layer (from the original 76.7 megabytes).

Alpine Linux variant

Our Ubuntu base image adds the most megabytes to our image (approx 113MB). Alpine Linux provides a popular alternative base in that is around 4 megabytes. It's based on altenative glibc implementation musl and busybox binaries, so not all software run well (or at all) with it, but our python container should run just fine. We'll create the following Dockerfile.alpine file:

FROM alpine:3.13

WORKDIR /usr/videos


RUN apk add --no-cache curl python3 ca-certificates && \
    curl -L -o /usr/local/bin/youtube-dl && \
    chmod a+x /usr/local/bin/youtube-dl && \
    apk del curl && \
    adduser -D userapp

USER userapp

ENTRYPOINT ["/usr/local/bin/youtube-dl"]



  • The package manager is apk and it can work without downloading sources (caches) first with --no-cache.
  • useradd is missing, but adduser exists.
  • Most of the package names are the same - there's a good package browser at

Now when we build this file with :alpine-3.13 as the tag:

$ docker build -t youtube-dl:alpine-3.13 -f Dockerfile.alpine .

It seems to run fine:

$ docker container run -v "$(pwd):/usr/videos" youtube-dl:alpine-3.13

From the history we can see that the our single RUN layer size is 39.4MB

$ docker image history youtube-dl:alpine-3.13

  14cfb0b531fb        20 seconds ago         /bin/sh -c apk add --no-cache curl python ca…   39.4MB
  <missing>           3 weeks ago         /bin/sh -c #(nop) ADD file:093f0723fa46f6cdb…   5.61MB

So in total our Alpine variant is about 45 megabytes, significantly less than our Ubuntu based image.

Back in part 1 we published the ubuntu version of youtube-dl with tag latest.

We can publish both variants without overriding the other by publishing them with a describing tag:

$ docker image tag youtube-dl:alpine-3.13 <username>/youtube-dl:alpine-3.13
$ docker image push <username>/youtube-dl:alpine-3.13

OR, if we don't want to upkeep the ubuntu version anymore we can replace our Ubuntu image by pushing this as the latest. Someone might depend on the image being ubuntu though.

$ docker image tag youtube-dl:alpine-3.13 <username>/youtube-dl
$ docker image push <username>/youtube-dl

Also remember that unless specified the :latest tag will always just refer to the latest image build & pushed - that can basically contain anything.

Multi-stage builds

Multi-stage builds are useful when you need some tools just for the build but not for the execution of the image CMD. This is an easy way to reduce size in some cases.

Let's create a website with Jekyll, build the site for production and serve the static files with nginx. Start by creating the recipe for Jekyll to build the site.

FROM ruby:3

WORKDIR /usr/app

RUN gem install jekyll
RUN jekyll new .
RUN jekyll build

This creates a new Jekyll application and builds it. We could start thinking about optimizations at this point but instead we're going add a new FROM for nginx, this is what resulting image will be. And copy the built static files from the ruby image to our nginx image.

FROM ruby:3 as build-stage
FROM nginx:1.19-alpine

COPY --from=build-stage /usr/app/_site/ /usr/share/nginx/html

This copies contents from the first image /usr/app/_site/ to /usr/share/nginx/html Note the naming from ruby to build-stage. We could also use external image as a stage, --from=python:3.7 for example. Lets build and check the size difference:

$ docker build . -t jekyll
$ docker image ls
  REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
  jekyll              latest              5f8839505f37        37 seconds ago      109MB
  ruby                latest              616c3cf5968b        28 hours ago        870MB

As you can see, even though our jekyll image needed ruby during the build process, its considerably smaller since it only has nginx and the static files. docker container run -it -p 8080:80 jekyll also works as expected.

Often the best choice is to use a FROM scratch image as it doesn't have anything we don't explicitly add there, making it most secure option over time.

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