Relocates the EVT sidecar container build context from the repo-root
evt/ directory into backend/app/plugins/evt/container/, so every
plugin (native or non-native) is self-contained under its own
subdirectory in the plugins tree.

No functional change. The runtime URL layout (/evt/{slug}/...) is
unaffected; only the build-time location of the Dockerfile +
nginx.conf changes.