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.

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:

• Github

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