cmPurgeBackup

I fixed first Dockerfile:
/# ARG IMAGE=intersystemsdc/iris-community:2019.4.0.383.0-zpm  seems broken

/# I used instead
ARG IMAGE=intersystemsdc/iris-community:2020.4.0.524.0-zpm

PULL Req. is posted.

The app is easy to handle and installs without problems.
example of the view in messages.log after 3 backup runs:

05/18/21-11:12:33:572 (1390) 0 [Generic.Event] INTERSYSTEMS IRIS JOURNALING SYSTEM MESSAGE
Journaling switched to: /usr/irissys/mgr/journal/20210518.005
05/18/21-11:12:47:685 (1390) 0 [Utility.Event] DELETE: /usr/irissys/mgr/journal/20210518.002
05/18/21-11:15:01:046 (1440) 0 [Generic.Event] INTERSYSTEMS IRIS JOURNALING SYSTEM MESSAGE
Journaling switched to: /usr/irissys/mgr/journal/20210518.006
05/18/21-11:15:31:453 (1440) 0 [Utility.Event] DELETE: /usr/irissys/mgr/journal/20210518.003
05/18/21-11:16:00:996 (1474) 0 [Utility.Event] () cm.Task.PurgeBackup: Purge Backup v. 2.6.21 -- Started: 18 May 2021 11:16:00
05/18/21-11:16:00:996 (1474) 0 [Utility.Event] (\*\*\*) cm.Task.PurgeBackup: DirsFromSYS -- LASTFULL: 2021-05-18 11:15:01\~/usr/irissys/mgr/Ba
ckup/FullDBList\_20210518\_006.cbk
05/18/21-11:16:00:998 (1474) 0 [Utility.Event] (\*\*\*) cm.Task.PurgeBackup: DirsSchedSpace -- Backup tasks directory(ies). Currently free =
230.881 GB
05/18/21-11:16:01:000 (1474) 0 [Utility.Event] (\*\*\*) cm.Task.PurgeBackup: FilesDel -- FullBackupsToKeep = 1, wanted FreeSpace = .16 GB
05/18/21-11:16:01:000 (1474) 0 [Utility.Event] (\*\*\*) cm.Task.PurgeBackup: FilesDel -- Passed 2021-05-18 11:15:01\~/usr/irissys/mgr/Backup/
FullDBList\_20210518\_006.cbk\~Completed\~Full
05/18/21-11:16:01:007 (1474) 0 [Utility.Event] (\*\*\*) cm.Task.PurgeBackup: FilesDel -- Deleted 2021-05-18 11:12:33\~/usr/irissys/mgr/Backup/
FullDBList\_20210518\_005.cbk\~Completed\~Full
05/18/21-11:16:01:015 (1474) 0 [Utility.Event] (\*\*\*) cm.Task.PurgeBackup: FilesDel -- Deleted 2021-05-18 10:54:01\~/usr/irissys/mgr/Backup/
FullDBList\_20210518\_004.cbk\~Completed\~Full
05/18/21-11:16:01:017 (1474) 0 [Utility.Event] (\*\*\*) cm.Task.PurgeBackup: Purge Backup v. 2.6.21 -- Finished: 18 May 2021 11:16:01 Currently free = 231.047 GB; **2 backup file(s) deleted, 1 full backup(s) kept**