[docker] How do I setup OpenRC in Docker?

[SamuelMarks]

Followed this answer https://stackoverflow.com/a/78270127

A container doesn't really "boot". You simply choose what process
starts when the container starts -- maybe that's a process manager,
like openrc, but more generally it's a specific service.

If we're designing a container to work with openrc, we could start
with something like this:

FROM docker.io/alpine:latest

RUN apk add openrc mdevd-openrc
RUN sed -i '/getty/d' /etc/inittab

CMD ["/sbin/init"]

A container that runs a couple of services under the control of openrc
might look like this:

FROM openrc-base

RUN apk add openssh-server openssh-server-common-openrc darkhttpd darkhttpd-openrc 
RUN rc-update add sshd default && rc-update add darkhttpd default

RUN mkdir -p -m 700 /root/.ssh
COPY id_rsa.pub /root/.ssh/authorized_keys
RUN chmod 600 /root/.ssh/authorized_keys

If we build an image from that Dockerfile and run it...

docker run --rm -p 2200:22 -p 8080:80 openrc-multiservice

...we'll see output like this:

• /proc is already mounted * /run/openrc: creating directory *
  /run/lock: correcting mode * /run/lock: correcting owner OpenRC
  0.52.1 is starting up Linux 6.7.10-200.fc39.x86_64 (x86_64)

• Caching service dependencies ... [ ok ] * /var/log/darkhttpd:
  correcting owner * Starting darkhttpd web server ... [ ok ]
  ssh-keygen: generating new host keys: RSA ECDSA ED25519 * Starting
  sshd ... [ ok ]

No errors, I didn't have to enable "softlevel" mode, and our services
start up as expected. But maybe someone only wants to run a single
service without using openrc! In that case, they can override CMD on
the command line:

docker run --rm -p 8080:80 openrc-multiservice darkhttpd /etc --port
80

That works, too.

In general, images based on our openrc image should be designed with
the fact that openrc is an option in mind (they should install rc
files for any provided services, etc), but they can have any
ENTRYPOINT script they want as long as it respects CMD and ultimately
starts /sbin/init.

At the same time, they can support single-service instances as well.

But got this error:

 * /proc is already mounted
 * Mounting /run ...mount: permission denied (are you root?)

 * Unable to mount tmpfs on /run.

 * Can't continue.
   OpenRC 0.55.1 is starting up Linux 6.10.14-linuxkit (aarch64) [DOCKER]

 * Caching service dependencies ... [ ok ]
mkdir: can't create directory '/sys/fs/cgroup/openrc.sshd': Read-only file system
 * Starting sshd ... [ ok ]

I am running Docker version 28.0.1, build 068a01e on macOS (aarch64)