This will simplify the ci and unify the way we manage docker image locally.

This splits the current test image script in 2:
- one to (locally) build and test the image
- one to only test the image

This introduces a Makefile to trigger simple test targets intended to be used by the ci.

As an implementation (and gory) detail, this opens a conditional within the python
script to not do anything during the download phase when the base url starts with
test://. While i'm not a fan, it's simple enough to allow this `make test` to work
without too much complication.

Related to T3746

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

Related to T4105

The /opt/extract_indexes.sh is not parametric and hard-codes the path to the
$WORKDIR/nexus-maven-repository-index.gz. And the remaining part of the python scripts
also uses that hardcoded name. So unify a bit and drop unnecessary instructions.

This adapts the existing script to:
- use click which autodocuments the cli
- add default values for considered less important parameters than the base-url
- switch to logging instead of print statements
- allows to provide another image (default to maven-index-exporter).

This also adapts the documentation about the script accordingly.

- wrap lines to 80
- clean up whitespace characters
- ...