gitignore log files from docker

[tomholub]

I think log files could definitely be gitignored, it's logs after all. Won't break anything. But I'll merge it.

Originally posted by @tomholub in #1601 (comment)

[DenBond7]

@tomholub @ioanmo226 I've done a small research. It looks like it's not a good idea to skip these files

For history

1. Due to It is not possible to work with this repository on Windows #1076, we had to change DOVECOT_MAILBOX_FORMAT to sdbox(was maildir). A new mail format uses different logic to handle message history. Based on dbox Mailbox Format

2. Also I've found a conversation where developers don't recommend skipping these files.
3. ref Mail index file format