containerize kenaan (i.e. commit_bot)

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Trac currently talks to kenaan via these lines in its own config:

trac-env/conf/trac.ini:message_executable = /srv/kenaan/bin/message
trac-env/conf/trac.ini:ticket_executable = /srv/kenaan/bin/ticket

which means it needs the "software" of kenaan installed as well, but luckily these are just straightforward network clients and can happily talk to a separate container.