fix fonts build with upstream dependent make vars

[benaryorg]

The upstream build depends on Docker in that it will use default rules when run normally, then execute a container, and in that container check an environment variable to execute the actual build, dependant on how it's called. TL;DR: CONTAINER=1 fixes the build. This also cleans up YAML syntax for multiline as well as using a single make call rather than the ephemeral file.

Partial fix for #150.

[flathubbot]

Started test build 38577

[flathubbot]

Build 38577 failed

[Lctrs]

Manually merged. Thanks.