Make LDAP configurable in the container image

[jcgruenhage]

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[jcgruenhage]

Once again don't get how the CI failure is related to the PR. From my testing, this works, I just logged into a server using this.

[richvdh]

I recently rejected #3284 on the grounds that adding a bajillion environment vars to the container setup wasn't scalable and at some point people should just write a yaml file themselves.

I'm willing to hear reasons that it's a good idea in this case though?

[and yes, the CI failure looks unrelated...]

[jcgruenhage]

I agree that this isn't scalable, but I don't agree that writing the config file myself is the solution here. Other tools are completely configurable via env vars (see grafana for example: http://docs.grafana.org/installation/configuration/#using-environment-variables), having similar possibilities in Synapse would be nice.

This template stuff was one of the things I criticized back when the docker PR wasn't merged yet. Let's not merge this anyway, you are right that this is getting too large.