Package Index: Build in Dockerhub (#13810)

* Package Index: Build in Dockerhub

Prepare to build the package index service, packages.spack.io,
on Dockerhub.

Local build (in spack root dir):
```
docker build -t spack/packages.spack.io:latest -f share/spack/docker/package-index/Dockerfile .
```

Local test:
```
docker run -p 8080:80 spack/packages.spack.io:latest
```

* Travis-CI: Remove Docker

Remove leftover docker stages from Travis-CI.

* Simplify Split Call