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Minimal Ubuntu images for Docker

Ever wondered why Ubuntu for Docker comes with systemd and tools for filesystem management? Yeah, me too. These are container images without that fuzz.

size comparison: Ubuntu for Docker 80 MiB, Ubuntu newgen 60 MiB, yet this one is 40 MiB

Features

  • small:
    • 63% the size of ubuntu-debootstrap (:16.04@898cb62b7368)
    • 45% the size of ubuntu (:16.04@44776f55294a)
  • comes with apt-transport-https
  • and latest curl
  • a bootstrap ca-certificates.crt
  • latest signify for Linux from Blitznote/signify
  • bzip2, jq, plzip, runit (for its chpst), unzip
  • with locale ISO.UTF-8 as default

Usage

This is meant as drop-in replacement for FROM ubuntu and FROM ubuntu-debootstrap.

You can use curl right away or start with apt-get -q update as usual. HTTPS support is already included in apt!

Find examples here:

Recommendations

Use lightweight chpst (31 kB) instead of gosu (2635 kB):

- gosu myuser syncthing "$@"
+ chpst -u myuser -- syncthing "$@"
- gosu nobody:root bash -c 'whoami && id'
+ chpst -u nobody:root -- bash -c 'whoami && id'

To account for differences between gpg v1 and gpg v2 I've created a script for fetching keys from keyservers:

/usr/bin/get-gpg-key 0xcbcb082a1bb943db 0xa6a19b38d3d831ef \
| apt-key add

Regenerate the Images

  1. Use the packages sources from /etc/apt/sources.list.
  2. Install the packages listed in build.manifest using apt.
  3. Remove any excess that cannot be used from within an container.

I have published a script which automates that. You can find it on Github, as answer to question/issue 2.

Hints

Leveraging git's "textconv" you can track changes to the archive files. This merely changes how diffs are displayed. Run:

git config --global diff.tar.textconv "tar -tavf"
git config --global diff.tar.cachetextconv true

git config --global diff.debian-tarball.textconv "tar --to-stdout -x ./var/lib/dpkg/available -f"
git config --global diff.debian-tarball.cachetextconv true

You can use screen and tmux, for example for long-running processes on Container Linux distributions. Run something that does not return first. Then, utilize script with docker exec -ti:

# on the host:
docker run -d --name "myenv" blitznote/apt-image:16.04 /bin/bash

# Now use this "permanent environment" like this:
docker exec -ti myenv script -q -c "/bin/bash" /dev/null

# Voila! screen/tmux will work as usual, including redraws on resized terminals.
screen -m -- rtorrent
screen -wipe
screen -r
…

Caveats

  • Images for architecture amd64/x86_64 require instruction set SSE 4, which has been introduced in 2007.
    If you don't have a reasonably recent CPU you will eventually run into the illegal instruction error. Another symptom of missing instruction sets is message Sub-process… exited unexpectedly with its accompanying logline trap invalid opcode (run dmesg or check your syslog daemon for details).
  • CPUs preceding AMD family 15h and Intel's Ivy Bridge will not work.
    Intel Edison as well as KNL is supported.
  • You need Linux 4.13.0 or later, with Seccomp for sandboxing of processes.
    zgrep SECCOMP /proc/config.gz