fix(ci): stage native binary in build-output/ to bypass .dockerignore

The .dockerignore has 'target' which excludes the target/
directory from the docker build context. The Dockerfile was
COPYing the binary from target/, so docker build failed with
'file not found in build context or excluded by .dockerignore'.

Fix: copy the binary to build-output/ (a non-excluded path)
before docker build, and update the Dockerfile to copy from
build-output/.

- Add 'Stage binary for Docker' step that does:
    mkdir -p build-output
    cp target/shot-crafter-calculator-1.0.0-runner build-output/
- Dockerfile COPY now reads build-output/shot-crafter-calculator-1.0.0-runner
- build-output/ is not in .dockerignore -> only the binary
  (~117MB) ships in the build context, not the whole target/
  tree (~200MB with classes, generated-sources, node binaries, etc.)

Dockerfile

@@ -1,6 +1,6 @@
 FROM quay.io/quarkus/quarkus-micro-image:2.0
 WORKDIR /work/
-COPY target/shot-crafter-calculator-1.0.0-runner /work/application
+COPY build-output/shot-crafter-calculator-1.0.0-runner /work/application
 
 RUN chmod 775 /work /work/application \
   && chown -R 1001 /work \