Add CHANGELOG.md, serve it in-app at /docs/changelog (closes theta42/theta-env#43)

GitHub Releases already carried real changelog notes per tag, but
those require internet access to view -- exactly what the /docs
route exists to avoid. CHANGELOG.md is a committed, Keep-a-Changelog
style file (backfilled from the v1.1.0/v1.1.1/v1.1.2 release notes),
linked from README and served at /docs/changelog alongside the rest
of the project's docs.

Dockerfile.openldap

@@ -98,6 +98,7 @@ COPY tos.md /tos.md
 # Documentation, served in-app at /docs (routes/docs.js) so it's readable
 # without internet access. Same flattened-path convention as tos.md above.
 COPY README.md /README.md
+COPY CHANGELOG.md /CHANGELOG.md
 COPY DEPLOYMENT.md /DEPLOYMENT.md
 COPY API.md /API.md
 COPY directory_spec.md /directory_spec.md