[vote-decrypt] Investigate mounted files & permission errors

[jsangmeister]

Currently, the dev setup in the feature/vote-decrypt branch mounts the votesdata folder from the vote-decrypt container into the test setup. This is not optimal as it leads to permission errors and unwanted side effects. It should be investigated why this is the case and then documented and/or improved, preferably with a solution which does not need access to internal files of the vote decrypt service.