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

@@ -113,6 +113,7 @@ COPY nodejs/api.md ./api.md
 # flattened /app (mirrors sso-manager-node's tos.md -> /tos.md convention);
 # docs/ mirrors the repo's own top-level docs/ folder.
 COPY README.md /README.md
+COPY CHANGELOG.md /CHANGELOG.md
 COPY DEPLOYMENT.md /DEPLOYMENT.md
 COPY docs /docs