No need for indirection.
In effect, this allows to push a standalone image in our docker registry.
That means all computations are done within the image and then that image
exposes only the result that needs to be published for the lister to consume.
Example use:
```
$ docker volume create maven_index_exporter
$ docker run \
-v maven_index_exporter:/work \
-v /tmp/maven_index_exporter:/publish \
softwareheritage/maven-index-exporter
$ file /tmp/maven_index_exporter/export.fld
/tmp/maven_index_exporter/export.fld: UTF-8 Unicode text, with very long lines
```
Related to T3746