fix(docker): drop RUN commands - quarkus-micro-image:2.0 has no shell

Removed the RUN chmod/chown/echo steps from the Dockerfile. The Quarkus
micro image 2.0 ships without /bin/sh and /usr/bin/sh, so any RUN
instruction fails. The chmod and securerandom.source append now happen
in the CI 'Stage binary for Docker' step, and COPY --chown=1001:1001
takes ownership of the binary in the image.

COPY --chown requires BuildKit, so:
- Install docker-buildx in the CI
- Switch docker build -> docker buildx build

The final image stays minimal (no shell, no microdnf, no extra packages).

Dockerfile

@@ -9,15 +9,10 @@ RUN microdnf install -y curl-minimal && microdnf clean all \
  && cp -rP /etc/ssl /out/etc/
 
 FROM quay.io/quarkus/quarkus-micro-image:2.0
-SHELL ["/usr/bin/sh", "-c"]
 WORKDIR /work/
-COPY build-output/shot-crafter-calculator-1.0.0-runner /work/application
+COPY --chown=1001:1001 build-output/shot-crafter-calculator-1.0.0-runner /work/application
 COPY --from=curl-builder /out/ /
 
-RUN chmod 775 /work /work/application \
- && chown -R 1001 /work \
- && echo "securerandom.source=file:/dev/urandom" >> /work/application
-
 EXPOSE 8080
 USER 1001