That's because there is no 'tag' for armhf according to the github page the image supports arm, therefore a 'tag' is not necessary.
This is your problem;
standard_init_linux.go:211: exec user process caused "exec format error"
why? because OMV does not have a PGID 1000, and do you have a user that has PUID 1000
Create a user for docker deployment, you do that in the omv's webui, that user is added to the user group, once done ssh into omv and enter id <username> this will return the UID and GID for that user, then use that in the stack.
BTW why are you adding plex: and creating a bridge network? there is no network specified on the docker or github page so one would assume this is using host.
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Actually, we were both right. This image doesn't support arm.
But the docker page here : https://hub.docker.com/r/digitalhigh/flextv
Says it does, but if you then look under the tags on the docker page there is only a x86 tag.
The following image is supposed to support armhf, but if you look at the image that you download, it is the same one as above. I have never seen this before, so it is very confusing. My guess is he just copied the directions from the above docker then pasted them in his own docker page.
https://hub.docker.com/r/theomajigga/flextv_pi
Corrected, it looks like it should be like this:
docker create \
--name=FlexTV \
-e PUID=1000 \
-e PGID=100 \
-e TZ=Europe/Madrid \
-p 6060:80 \
-v /srv/dev-disk-by-label-4TB/appdata/config/flextv:/config \
--restart unless-stopped \
theomajigga/flextv_pi:armhf
You can paste the above into a terminal. Will not work in stacks on portainer. If you want to use stacks, you will have to translate it.
And as Geaves said, don't forget to use your uid and pid. Create a user that has docker in its permissions. Using pi on a raspberry pi is a lot like using root on a regular server from what I understand, so not very good from a security perspective.