Prevents untracked files from being part of the cache key

There were no matched files for the previous pattern, see
https://github.com/ComPWA/polarimetry/actions/runs/3962250099/jobs/6788669966

Removes the need to run `apt install`

Dockerfile:

```Dockerfile
FROM ubuntu:22.04
RUN apt-get -y update

\# https://serverfault.com/a/992421
RUN DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt-get -y install tzdata
RUN apt-get -y install inkscape latexmk make texlive-fonts-extra texlive-xetex xindy

RUN apt-get clean
RUN rm -rf /var/lib/apt/lists/*
```

and uploaded it to Docker Hub:

```shell
sudo docker build -t compwa/polarimetry-fast-pdf:v1 .
sudo docker tag compwa/polarimetry-fast-pdf:v1 compwa/polarimetry-fast-pdf:v1
sudo docker push compwa/polarimetry-fast-pdf:v1
```

…container

This allows pinning to a specific Docker container