The latest versions of Docker for Windows and Docker for Mac can build non-native architecture containers. The Author (computermouth) reached out to Anil Madhavapeddy, an engineer at Docker, asking about why it wasn't the case on Linux, turns out Linux can too, there's just an issue with some distributions of qemu-user-static that's disallowing the functionality. These files enable (likely among other things) the ability to build arm32v7/* Docker images on a Linux host of a different architecture without putting qemu-*-static binaries in the image itself. (Only tested on Ubuntu 18.04 LTS x86_64)
Run the following command:
systemctl status systemd-binfmt
If you see output like the following [note the start condition failed part]:
user@ubuntu:~# systemctl status systemd-binfmt
● systemd-binfmt.service - Set Up Additional Binary Formats
Loaded: loaded (/lib/systemd/system/systemd-binfmt.service; static; vendor preset: enabled)
Active: inactive (dead)
Condition: start condition failed at Tue 2019-12-24 17:09:25 CET; 30s ago
├─ ConditionDirectoryNotEmpty=|/lib/binfmt.d was not met
├─ ConditionDirectoryNotEmpty=|/usr/lib/binfmt.d was not met
├─ ConditionDirectoryNotEmpty=|/usr/local/lib/binfmt.d was not met
├─ ConditionDirectoryNotEmpty=|/etc/binfmt.d was not met
└─ ConditionDirectoryNotEmpty=|/run/binfmt.d was not met
Docs: man:systemd-binfmt.service(8)
man:binfmt.d(5)
https://www.kernel.org/doc/Documentation/binfmt_misc.txt
...you will need the files here in one of the above directories. Personally, I've been using them in /lib/binfmt.d.
git clone https://github.com/eagle-1/qemu-static-conf.git
sudo mkdir -p /lib/binfmt.d
sudo cp qemu-static-conf/*.conf /lib/binfmt.d/
sudo systemctl restart systemd-binfmt.service
NOTE: You will need to sudo docker below if you haven't set up the appropriate permissions. You might have to restart docker service for buildx to pick up the additional platforms.
user@ubuntu:~# docker buildx ls
NAME/NODE DRIVER/ENDPOINT STATUS PLATFORMS
testbuilder * docker-container
testbuilder0 unix:///var/run/docker.sock running linux/amd64, linux/arm64, linux/s390x, linux/386, linux/arm/v7, linux/arm/v6
default docker
default default running linux/amd64, linux/arm64, linux/s390x, linux/386, linux/arm/v7, linux/arm/v6
user@ubuntu:~# docker run --rm -t arm32v6/alpine uname -m
armv7l
user@ubuntu:~# docker run --rm -t arm64v8/alpine uname -m
aarch64
If the container outputs its arch without error, you're good to go!
Fedora supplies these files with their qemu-user-static package, while Debian and Ubuntu don't. Bug report is here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=868217