release_tool: Fix non-docker-compose YAML files being picked up.
release_tool: Fix non-docker-compose YAML files being picked up.

This already works in the filesystem version of this detection, but
when querying the repository, we were not checking for the
"docker-compose" string.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>

GregorioDiStefano/integration,pasinskim/integration,pasinskim/integration,pasinskim/integration,GregorioDiStefano/integration