Change Docker base images from Ubuntu to Debian, and use `snapshot.debian.org` as apt source list.
Ubuntu occasionally removes version-pinned packages from apt (see #7484), which
- breaks historical reproducible builds
- introduces maintenance burden as we have to update the version pins
Hopefully this change fixes both issues.
merges https://github.com/spesmilo/electrum/pull/7926
closes https://github.com/spesmilo/electrum/issues/7484
-----
closes https://github.com/spesmilo/electrum/issues/6936
Note: if we used the python crowdin client (https://github.com/crowdin/crowdin-api-client-python),
we would do something like this:
```
from crowdin_api import CrowdinClient
class MyCrowdinClient(CrowdinClient):
TOKEN = ...
client = MyCrowdinClient()
with open(locale_file_name, 'rb') as f:
resp = client.storages.add_storage(f)
storage_id = resp['data']['id']
client.source_files.update_file(projectId=crowdin_project_id, storageId=storage_id, fileId=crowdin_file_id)
client.translations.build_crowdin_project_translation(projectId=crowdin_project_id)
```
