.dockerignore: ignore more directories
.dockerignore: ignore more directories

New directories to ignore:

* backup - a non-version-controlled data dump directory
* opendkim - a version-controlled directory that is volume-mounted

hemberger/smr,hemberger/smr,smrealms/smr,smrealms/smr,smrealms/smr,hemberger/smr,hemberger/smr