Local run on my machine does not work properly. This generates pkg-resources==0.0.0 and
probably some other irrelevant things.

So, instead, I used docker:

```
$ docker run -ti -v $PWD:/src -u root --entrypoint=bash python:3.9
```

In another shell:

```
$ docker exec --user 1000 \
  $container_name /src/scripts/generate-frozen-requirements swh-graphql
...
```
- $container_name is derived out of `docker ps`.
- uid 1000 is derived out of your `id $USER`

Related to T4464

It makes impossible to run apt after, which can make the debugging
complicated if some packages need to be added during a debug session

It's not necessary in a immutable environment

Related to T4134

As it's the first application, everything is in a uniq Dockerfile.
We'll see later if some parts can be factorized

Related to T4134

Related to T4134

Related to T4131