Arch Docker Baseimage

Problem

You need a base image of Arch Linux that you're sure is a-okay.

Compounding Factors

Base images currently can't be so-called 'Trusted Builds' built by the repository itself. You have no way of verifying the integrity of the images available.
Arch is very much a moving target distribution. Any images you do pull are likely to be out of date.
You don't have an existing Arch Linux install, which the current scripts require. and don't want to make one, or use someone else's container or install to build your image, again, going back to trust.
The Arch Netinstall ISO doesn't have enough disposable storage to run the current scripts.

Solution

Dependencies

You'll need:

gpg
curl
docker

Get an Arch Chroot

On any 64-bit Linux 2.6.23 or later you can create your base image via a chroot.

# Make some space
mkdir archbuild
cd archbuild
# Get the Image
VERSION=$(curl https://mirrors.kernel.org/archlinux/iso/latest/ | grep -Poh '(?<=archlinux-bootstrap-)\d*\.\d*\.\d*(?=\-x86_64)' | head -n 1)
curl https://mirrors.kernel.org/archlinux/iso/latest/archlinux-bootstrap-$VERSION-x86_64.tar.gz > archlinux-bootstrap-$VERSION-x86_64.tar.gz
curl https://mirrors.kernel.org/archlinux/iso/latest/archlinux-bootstrap-$VERSION-x86_64.tar.gz.sig > archlinux-bootstrap-$VERSION-x86_64.tar.gz.sig
# Pull Pierre Schmitz PGP Key.
# http://pgp.mit.edu:11371/pks/lookup?op=vindex&fingerprint=on&exact=on&search=0x4AA4767BBC9C4B1D18AE28B77F2D434B9741E8AC
gpg --keyserver pgp.mit.edu --recv-keys 9741E8AC
# Verify its integrity.
gpg --verify archlinux-bootstrap-$VERSION-x86_64.tar.gz.sig

Check the output, make sure the signature is good. You'll likely see a trust warning, that's fine, you didn't mark the key trusted.

Getting In

# Extract
tar xvf archlinux-bootstrap-$VERSION-x86_64.tar.gz
# Hop in
sudo ./root.x86_64/bin/arch-chroot root.x86_64

You should notice you shell looks like sh-4.3#. Furthermore, pacman should be available.

Setup

In the chroot:

# Setup a mirror.
echo 'Server = https://mirrors.kernel.org/archlinux/\$repo/os/\$arch' > /etc/pacman.d/mirrorlist
# Setup Keys
pacman-key --init
pacman-key --populate archlinux
# Base without the following packages, to save space.
# linux jfsutils lvm2 cryptsetup groff man-db man-pages mdadm pciutils pcmciautils reiserfsprogs s-nail xfsprogs vi
pacman -Syu --noconfirm bash bzip2 coreutils device-mapper dhcpcd gcc-libs gettext glibc grep gzip inetutils iproute2 iputils less libutil-linux licenses logrotate psmisc sed shadow sysfsutils systemd-sysvcompat tar texinfo usbutils util-linux which
# Pacman doesn't let us force ignore files, so clean up.
pacman -Sc --noconfirm
# Install stuff
echo "en_US.UTF-8 UTF-8" > /etc/locale.gen
locale-gen
exit

Outside of the chroot (You may need to be root):

# udev doesnt work in containers, rebuild /dev
# Taken from https://raw.githubusercontent.com/dotcloud/docker/master/contrib/mkimage-arch.sh
DEV=root.x86_64/dev
rm -rf $DEV
mkdir -p $DEV
mknod -m 666 $DEV/null c 1 3
mknod -m 666 $DEV/zero c 1 5
mknod -m 666 $DEV/random c 1 8
mknod -m 666 $DEV/urandom c 1 9
mkdir -m 755 $DEV/pts
mkdir -m 1777 $DEV/shm
mknod -m 666 $DEV/tty c 5 0
mknod -m 600 $DEV/console c 5 1
mknod -m 666 $DEV/tty0 c 4 0
mknod -m 666 $DEV/full c 1 7
mknod -m 600 $DEV/initctl p
mknod -m 666 $DEV/ptmx c 5 2
ln -sf /proc/self/fd $DEV/fd

Import the Image

Outside of the chroot (You may need to be root):

USER='hoverbear'
tar --numeric-owner -C root.x86_64 -c .  | docker import - $USER/archlinux

Test It

docker run --rm=true -t -i $USER/archlinux /bin/bash

Give it a try!

A full, start to end script

You can grab a fully automated script here.