fix(ci): don't ignore docker-context/ so Dockerfile COPY works

The .dockerignore was excluding docker-context/, but the CI pipeline
creates docker-context/plugin/ after unzipping the plugin ZIP and the
Dockerfile COPYs from that path. Buildx was failing with:
  failed to calculate checksum of ref ...: "/docker-context/plugin":
  not found

Also removed gradle/wrapper/gradle-wrapper.jar from .dockerignore —
it's harmless in the image but there's no reason to block it.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: junseon-yoo

.dockerignore

@@ -8,6 +8,6 @@ build/classes/
 build/resources/
 build/generated/
 !build/distributions/
-docker-context/
-gradle/wrapper/gradle-wrapper.jar
+# docker-context/plugin/ is created by the CI pipeline and consumed
+# by the Dockerfile — must NOT be ignored.
 *.iml