installer: override the host's PATH with Alpine's default (#901)
installer: override the host's PATH with Alpine's default (#901)

We override the environment variables PATH of the host system with the
default of Popper’s Docker image. This prevents unforeseeable side
effects if the host’s PATH is non-standard.

I obtained the default PATH from the Popper image like this:

```
docker run --rm --entrypoint sh getpopper/popper -c printenv | grep PATH
```

[skip ci]

fixes #900
systemslab/popper,systemslab/popper