Update Vuepress version
The CI was building with an older version of Vuepress (previous to 1.0.0), and that was leading to errors when, e.g, making use of theme inheritance:
$ yarn run build
yarn run v1.5.1
$ vuepress build docs
Extracting site metadata...Error: [vuepress] Cannot resolve Layout.vue file in .vuepress/theme.
This problem was mentioned in this issue: https://github.com/vuejs/vuepress/issues/1216
Thanks for providing the example page. Take care (: