Make the docker image extract and publish the export.fld directly

The export.fld file is the interesting result of this repository for one maven indexa.
With this new commit, running the image will compute and expose directly that result to
be published.

Prior to this commit, the docker image was only in charge of triggering the
extract_index.sh script. The python script was the orchestrator in charge of downloading
indexes, run the docker image and out of the index files retrieved, extract the
resulting export.fld script.

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  # on the machine running docker
/tmp/maven_index_exporter/export.fld: UTF-8 Unicode text, with very long lines

Related to T3746