This will not be easy as part of a docker update. I believe and maybe everyone can chime in if I am wrong, but to upgrade to .28 within the container is not easy. The easiest and most realistic method would be to shutdown your .27 instance and have a new .28 instance. In order to do an update to .28 within an existing docker would be really tough to accomplish. I also tried to do all of what you didl within the docker and was unsuccessful.
The issues I have run into is that the new .28 mythtv uses QT5 libraries. I was having issues with QT5 in conjunction with running the mythtv-setup application that Sparkly has so nicely setup via xrdp connection. The only way I was able to get around this is to create a new docker with Phusion .9.19 and mythtv .28. I am not an expert by any means, but it would way more of a challenge to upgrade. Would there be interest in a .28 docker container that would be all new, that you would have to re-go through the setup of your tuners, and channel data and also re-setup all your recording schedules? I know this is a pain. I know, I just went through it, but it is actually alot easier than going through an upgrade within the docker already created....
The main reason I wanted the .28 docker is that the Services API finally does not need to rely on StartTS and Channel ID to delete recordings via the API. I was playing with some python scripts I wrote to see if I could automatically move the Mythtv Recordings to the Directory structure I have for Plex to automatically add things to the library. I am a long ways off from that, but with .28, the dvr service api allows you delete recordings based on a recording ID unique to each recording instead of trying to keep track of StartTS and Channel ID.
Anyway, I am babbling....
Sparkly,
Do you have any issues with me creating a new .28 only docker based off of all of your hard work? I want to start contributing back since I have enjoyed your Mythtv Docker for the last year. It has been awesome! I just needed .28 so I went through a lot of the initial work to try and get it working in a docker based off of phusion .9.19 and your Dockerfile and init scripts. I have a fairly repeatable build out there on dockerhub as well as a templates repo build, but have kept my work as a private repo for community applications so far since I feel it isn't my place since it was all your hard work. Can I help you release a new .28 docker under your repo? I tried to private message you, but it said I was denied.
mp54u