Note: I now also implemented it trying to follow the usual chrony generic/OSType split as I hope this will make it more acceptable.
WIP Test PKG is available in PPA: https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/3188

Tests:
1. starts in container if -x is set in /etc/defaults/chrony - yes
2. implies -x if not able to control clock - yes
3. in an environment where it can control the time it does so (without fallback)- yes
4. -x in an environment where it can control the time works - TBD

#1
● chrony.service - chrony, an NTP client/server
   Loaded: loaded (/lib/systemd/system/chrony.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-03-07 11:15:23 UTC; 2s ago
     Docs: man:chronyd(8)
           man:chronyc(1)
           man:chrony.conf(5)
  Process: 30709 ExecStartPost=/usr/lib/chrony/chrony-helper update-daemon (code=exited, status=0/SUCCESS)
  Process: 30705 ExecStart=/usr/sbin/chronyd $DAEMON_OPTS (code=exited, status=0/SUCCESS)
 Main PID: 30707 (chronyd)
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/chrony.service
           └─30707 /usr/sbin/chronyd -x

Mar 07 11:15:23 b systemd[1]: Starting chrony, an NTP client/server...
Mar 07 11:15:23 b chronyd[30707]: chronyd version 3.2 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SECHASH +SIGND +ASYN
Mar 07 11:15:23 b chronyd[30707]: Disabled control of system clock
Mar 07 11:15:23 b chronyd[30707]: Frequency 0.000 +/- 1000000.000 ppm read from /var/lib/chrony/chrony.drift
Mar 07 11:15:23 b systemd[1]: Started chrony, an NTP client/server.


#2
Mar 07 11:16:45 b systemd[1]: Starting chrony, an NTP client/server...
Mar 07 11:16:45 b chronyd[30727]: 2018-03-07T11:16:45Z Time not adjustable, implying -x (do not set system clock)
Mar 07 11:16:45 b chronyd[30729]: chronyd version 3.2 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SECHASH +SIGND +ASYN
Mar 07 11:16:45 b chronyd[30729]: Disabled control of system clock
Mar 07 11:16:45 b chronyd[30729]: Frequency -5.126 +/- 50.533 ppm read from /var/lib/chrony/chrony.drift
Mar 07 11:16:45 b systemd[1]: Started chrony, an NTP client/server.

#3
● chrony.service - chrony, an NTP client/server
   Loaded: loaded (/lib/systemd/system/chrony.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-03-07 11:25:40 UTC; 12s ago
     Docs: man:chronyd(8)
           man:chronyc(1)
           man:chrony.conf(5)
 Main PID: 26894 (chronyd)
    Tasks: 1 (limit: 551)
   CGroup: /system.slice/chrony.service
           └─26894 /usr/sbin/chronyd

Mär 07 11:25:40 b-test systemd[1]: Starting chrony, an NTP client/server...
Mär 07 11:25:40 b-test chronyd[26894]: chronyd version 3.2 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SECHASH +SIGND 
Mär 07 11:25:40 b-test chronyd[26894]: Initial frequency -3.327 ppm
Mär 07 11:25:40 b-test systemd[1]: Started chrony, an NTP client/server.
Mär 07 11:25:53 b-test chronyd[26894]: Selected source 84.2.44.19

#4
● chrony.service - chrony, an NTP client/server
   Loaded: loaded (/lib/systemd/system/chrony.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-03-07 11:27:00 UTC; 1s ago
     Docs: man:chronyd(8)
           man:chronyc(1)
           man:chrony.conf(5)
  Process: 27065 ExecStartPost=/usr/lib/chrony/chrony-helper update-daemon (code=exited, status=0/SUCCESS)
  Process: 27060 ExecStart=/usr/sbin/chronyd $DAEMON_OPTS (code=exited, status=0/SUCCESS)
 Main PID: 27062 (chronyd)
    Tasks: 1 (limit: 551)
   CGroup: /system.slice/chrony.service
           └─27062 /usr/sbin/chronyd -x

Mär 07 11:27:00 b-test systemd[1]: Starting chrony, an NTP client/server...
Mär 07 11:27:00 b-test chronyd[27062]: chronyd version 3.2 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SECHASH +SIGND 
Mär 07 11:27:00 b-test chronyd[27062]: Disabled control of system clock
Mär 07 11:27:00 b-test chronyd[27062]: Frequency -3.327 +/- 1000000.000 ppm read from /var/lib/chrony/chrony.drift
Mär 07 11:27:00 b-test systemd[1]: Started chrony, an NTP client/server.


All that makes me feel good for a first patch submission to upstream.