You could put your SSH server configuration in a repo. You could put your SSH authorization key in a repo. You could even put your private key in a repo if you really wanted.
The nicety here on harbormaster seems to be that there are some ways to use the same code as a template in which specific differences are dynamically inserted by harbormaster. I'm not aware of how you could use docker-compose (without swarm) to accomplish this, unless you start doing a lot of bash stuff.
I also appreciate that harbormaster offers opinions on secrets management.
While I haven't used it personally, there is [0] Watchtower which aims to automate updating docker containers.
You run what's supposed to run the same way you would anything else. It's the same for the environment variables.
How would you track what's supposed to run and what's not for Docker? Using the `DOCKER_HOST` environment variable to connect over SSH is the exact same way.
I have never used Chef. This is babble to me.
In an imprecise nutshell: You specify what needs to exist on the target system using Chef's DSL and Chef client will converge the state of the target to the desired one.
To "converge" is to run something until it is stable. This terminology, I think, comes from the early configuration managemnt system CFEngine, where you write your configuration in declarative(-ish) "make it so this is true" steps, instead of imperative "perform this change" steps the way that a shell script would do. See e.g. https://www.usenix.org/legacy/publications/library/proceedin...
chef-solo is a command that executes Chef's client - the thing that actually makes configuration changes, that is to say, "converges cookbooks" - in a way that does not require a server component. The normal way of deploying Chef is that a server runs things on clients, the machines being configured, but chef-solo is appropriate for the case where there is no such distinction and there's just one machine where you wish to run Chef.
Agreed on it being a bit too heavy-handed, and the tooling isn't very helpful for dealing with it unless you're neck-deep into the ecosystem already.