Test a few more functionalities in the image. Change how postfix
message IDs are detected. Message ID can be set by the client and
hence detecting them via regex is not the best way to go around it.
This fix will actually look at the log line and try to determine
if we're looking at the message ID or not.
- It's now possible to choose the logging type - either 'plain' or
'json'
- The code is ready to support multiple integration tests (with
different configurations)
- `OPENDKIM_` and `POSTFIX_` variables are handled properly and recorded
in the corresponding files. (This had a downfall that `bash` now needs
to be installed, so we can probably simplify some of the shell
scripts.)
