Update ARM sources

[OniriCorpe]

Problem

• the arm sources are really really outsdated

Solution

• replaced it with this one https://github.com/remiheens/mattermost-docker-arm

PR Status

• [ ] Code finished and ready to be reviewed/tested
• [ ] The fix/enhancement were manually tested (if applicable)

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[kemenaran]

LGTM. The current armv7 sources are indeed heavily deprecated (because maintaining the build script to accommodate changes upstream is hard), so switching to more up-to-date sources sounds much better.

[kemenaran]

Thanks!