[3006.x] Docker systemd container identified as physical

[furlongm]

Description

The virtual and virtual_subtype grains in a systemd based Docker container shows as "physical" instead of "container". This seems to be because /proc/1/cgroup no longer contains the docker strings, which is what we test for:

# cat /proc/1/cgroup
0::/

Similar issue to #67733

Setup
(Please provide relevant configs and/or SLS files (be sure to remove sensitive info. There is no general set-up of Salt.)

Please be as specific as possible and give set-up details.

• on-prem machine
• VM (Virtualbox, KVM, etc. please specify)
• VM running on a cloud service, please be explicit and add details
• container (Kubernetes, Docker, containerd, etc. please specify)
• or a combination, please be explicit
• jails if it is FreeBSD
• classic packaging
• onedir packaging
• used bootstrap to install

Steps to Reproduce the behavior

# salt-call --local grains.get virtual
local:
    physical

# salt-call --local grains.get virtual_subtype
local:

Expected behavior

# salt-call --local grains.get virtual
local:
    container

# salt-call --local grains.get virtual_subtype
local:
    Docker

Screenshots
If applicable, add screenshots to help explain your problem.

Versions Report

salt --versions-report

(Provided by running salt --versions-report. Please also mention any differences in master/minion versions.)

# salt-call --versions-report
Salt Version:
          Salt: 3006.10

Python Version:
        Python: 3.10.16 (main, Mar  6 2025, 02:23:15) [GCC 11.2.0]

Dependency Versions:
          cffi: 1.14.6
      cherrypy: 18.6.1
  cryptography: 42.0.5
      dateutil: 2.8.1
     docker-py: Not Installed
         gitdb: Not Installed
     gitpython: Not Installed
        Jinja2: 3.1.6
       libgit2: Not Installed
  looseversion: 1.0.2
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.0.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     packaging: 22.0
     pycparser: 2.21
      pycrypto: Not Installed
  pycryptodome: 3.19.1
        pygit2: Not Installed
  python-gnupg: 0.4.8
        PyYAML: 6.0.1
         PyZMQ: 23.2.0
        relenv: 0.18.1
         smmap: Not Installed
       timelib: 0.2.4
       Tornado: 4.5.3
           ZMQ: 4.3.4

System Versions:
          dist: ubuntu 22.04.5 jammy
        locale: utf-8
       machine: x86_64
       release: 6.4.0-150600.23.33-default
        system: Linux
       version: Ubuntu 22.04.5 jammy

Additional context
PR for this issue is #67906