Dockerfile examples for containerized systemd (mainly for test environments)

• Dockerfile.archlinux: Arch Linux (systemd 247, as of Feb 2021)
• Dockerfile.centos-8.3: CentOS 8.3 (systemd 239)
• Dockerfile.debian-10: Debian GNU/Linux 10 (systemd 241)
• Dockerfile.fedora-33: Fedora 33 (systemd 246)
• Dockerfile.opensuse-tumbleweed: openSUSE Tumbleweed (systemd 246, as of Feb 2021)
• Dockerfile.ubuntu-20.04: Ubuntu 20.04 (systemd 245)

Demo 1: interactive shell with systemctl

• The command (/bin/bash) specified as the argument of docker run is executed as the foreground job in the container.
• Workdir (--workdir /usr) is propagated
• Env vars (-e FOO=hello) are propagated
• The container shuts down when the command exits. The exit status code (42) is propagated.

host$ docker build -t foo -f Dockerfile.debian-10 .
host$ docker run -it --rm --privileged --workdir /usr -e FOO=hello foo /bin/bash
Created symlink /etc/systemd/system/docker-entrypoint.target.wants/docker-entrypoint.service → /etc/systemd/system/docker-entrypoint.service.
Created symlink /etc/systemd/system/systemd-firstboot.service → /dev/null.
Created symlink /etc/systemd/system/systemd-udevd.service → /dev/null.
/docker-entrypoint.sh: starting /lib/systemd/systemd --show-status=false --unit=docker-entrypoint.target
systemd 241 running in system mode. (+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN -PCRE2 default-hierarchy=hybrid)
Detected virtualization docker.
Detected architecture x86-64.
Set hostname to <4608072355e2>.
+ /bin/bash
root@4608072355e2:/usr# systemctl status
● 4608072355e2
    State: running
     Jobs: 0 queued
   Failed: 0 units
    Since: Mon 2020-02-10 21:41:37 UTC; 3s ago
   CGroup: /system.slice/docker-4608072355e222fd25bdfa4b74e48c3f087f4d8128814a29d4035e7a8e42a364.scope
           ├─init.scope
           │ └─1 /lib/systemd/systemd --show-status=false --unit=docker-entrypoint.target
           └─system.slice
             ├─systemd-journald.service
             │ └─26 /lib/systemd/systemd-journald
             └─docker-entrypoint.service
               ├─38 /bin/sh -xec /bin/bash
               ├─39 /bin/bash
               ├─40 systemctl status
               └─41 pager
root@4608072355e2:/usr# pwd
/usr
root@4608072355e2:/usr# echo $FOO
hello
root@4608072355e2:/usr# exit 42
exit
host$ echo $?
42

Demo 2: journalctl -f

host$ docker run -it --rm --privileged foo journalctl -f
Created symlink /etc/systemd/system/docker-entrypoint.target.wants/docker-entrypoint.service → /etc/systemd/system/docker-entrypoint.service.
Created symlink /etc/systemd/system/systemd-firstboot.service → /dev/null.
Created symlink /etc/systemd/system/systemd-udevd.service → /dev/null.
/docker-entrypoint.sh: starting /lib/systemd/systemd --show-status=false --unit=docker-entrypoint.target
systemd 241 running in system mode. (+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN -PCRE2 default-hierarchy=hybrid)
Detected virtualization docker.
Detected architecture x86-64.
Set hostname to <827af95def7e>.
+ journalctl -f
-- Logs begin at Wed 2020-02-12 06:20:22 UTC. --
Feb 12 06:20:22 827af95def7e systemd[1]: Started Flush Journal to Persistent Storage.
Feb 12 06:20:22 827af95def7e systemd[1]: Starting Create Volatile Files and Directories...
Feb 12 06:20:22 827af95def7e systemd[1]: Started Create Volatile Files and Directories.
Feb 12 06:20:22 827af95def7e systemd[1]: Condition check resulted in Network Time Synchronization being skipped.
Feb 12 06:20:22 827af95def7e systemd[1]: Reached target System Time Synchronized.
Feb 12 06:20:22 827af95def7e systemd[1]: Starting Update UTMP about System Boot/Shutdown...
Feb 12 06:20:22 827af95def7e systemd[1]: Started Update UTMP about System Boot/Shutdown.
Feb 12 06:20:22 827af95def7e systemd[1]: Reached target System Initialization.
Feb 12 06:20:22 827af95def7e systemd[1]: Started docker-entrypoint.service (journalctl -f).
Feb 12 06:20:22 827af95def7e systemd[1]: Startup finished in 410ms.
^Cgot signal INT
host$ echo $?
130

Bugs

• docker run needs -t