Include paxtest in the image

Hi,

It would be convenient to have paxtest included in the image:

```
FROM ${DOCKER_IMAGE:-archivebox/archivebox:master}

RUN apt-get update && apt-get install -y \
paxctl && rm -rf /var/lib/apt/lists/*

RUN paxctl -cm /usr/bin/node
```

When running node applications in hardened environments (or building them in such environments) 'node' needs to be able to create memory mappings for JIT. It's a security trade-off, thus you can likely skip adding the paxctl step. This will make it easier to build/adjust the images in network-restricted environments.

Start by locating the Docker image build definition and the existing package-install steps, then compare them with the Dockerfile snippet in the issue. Determine whether adding paxctl or paxtest is intended, and verify that the resulting image supports the described Node.js use case without unnecessary package-cache files.

Written by the indexing model from the issue text.