Time machine with docker and samba, even on big sur. Three parts: docker-compose file, time machine config snippet (for in docker volume), and avahi service

docker-compose.yml

# ...
  samba:
    image: dperson/samba
    restart: unless-stopped
    command: >-
              -u "user;badpass"
              -u "timemachine_rmbp;${TIMEMACHINE_RMBP_PASSWORD}"
              -s "data;/media/data;yes;yes;yes;all"
    ports:
      - 139:139
      - 445:445
    environment:
      - USERID=1000
      - INCLUDE=/config/timemachine.conf
      - NMDB=true
      - TZ=Europe/Amsterdam
      - RECYCLE=false
      - WORKGROUP=workgroup
    volumes:
      - ./samba:/config:ro
      - /srv/timemachine/timemachine:/media/timemachine
 # ...

samba_timemachine.service

<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
 <name replace-wildcards="yes">%h</name>
 <service>
   <type>_adisk._tcp</type>
   <txt-record>sys=waMa=0,adVF=0x100</txt-record>
   <txt-record>dk0=adVN=time machine,adVF=0x82</txt-record>
 </service>
  <service>
    <type>_smb._tcp</type>
    <port>445</port>
  </service>
</service-group>

timemachine.conf

[global]
    fruit:nfs_aces = no
    inherit permissions = yes

    vfs objects = catia fruit streams_xattr acl_xattr

[time machine rmbp12]
    comment = Time Machine retina macbook 2012
    path = /media/timemachine/rmbp12
    browseable = yes
    read only = no
    guest ok = no
    writeable = yes

    veto files = /._*/.apdisk/.AppleDouble/.DS_Store/.TemporaryItems/.Trashes/desktop.ini/ehthumbs.db/Network Trash Folder/Temporary Items/Thumbs.db/
    delete veto files = yes
    valid users = timemachine_rmbp
    admin users = timemachine_rmbp

    create mask = 0600
    directory mask = 0700

    spotlight = yes
    # Some fruit settings
    fruit:aapl = yes
    fruit:encoding = native
    fruit:metadata = stream
    fruit:resource = xattr

    fruit:time machine = yes
    fruit:time machine max size = 300G