When the the system has dash installed and used as the default shell the postinst fails with an exit status 2. What appears to be the culprit is the dbmigrate script choking on POSIX compliance.

The particular segment causing an issue is 'USERNAME=$(/usr/bin/id -un)', it responds that the '(' is unexpected and it is expecting a '{'

A quick fix could be to change #!/bin/sh to #!/bin/bash, however this is likely to cause breakage.