fix(ci): Add missing OpenBao Dockerfile

The docker-build-openbao pipeline step was failing because the Dockerfile
was missing from docker/openbao/.

Created a minimal Dockerfile that:
- Uses official quay.io/openbao/openbao:2 as base
- Copies config.hcl and init.sh into the image
- Exposes port 8200
- Preserves the default entrypoint from base image

This allows Kaniko to build the stack-openbao image for Swarm deployment.

Fixes pipeline #325 docker-build-openbao failure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/openbao/Dockerfile

@@ -0,0 +1,19 @@
+FROM quay.io/openbao/openbao:2
+
+LABEL maintainer="Mosaic Stack <dev@mosaic.local>"
+LABEL description="OpenBao secrets management for Mosaic Stack"
+
+# Copy OpenBao configuration
+COPY config.hcl /openbao/config/config.hcl
+
+# Copy auto-initialization script
+COPY init.sh /openbao/init.sh
+RUN chmod +x /openbao/init.sh
+
+# Expose OpenBao port
+EXPOSE 8200
+
+# Use the default entrypoint from the base image
+# The container will be started with either:
+# - Default: openbao server -config=/openbao/config/config.hcl
+# - Init sidecar: /openbao/init.sh