fix: set executable permissions on binaries in Docker image

COPY without --chmod=755 preserves the source file permissions, which
may not include the execute bit depending on the build environment.
This caused /nebula: permission denied in distroless containers.

Author: privsim

docker/Dockerfile

@@ -1,8 +1,8 @@
 FROM gcr.io/distroless/static:latest
 
 ARG TARGETOS TARGETARCH
-COPY build/$TARGETOS-$TARGETARCH/nebula /nebula
-COPY build/$TARGETOS-$TARGETARCH/nebula-cert /nebula-cert
+COPY --chmod=755 build/$TARGETOS-$TARGETARCH/nebula /nebula
+COPY --chmod=755 build/$TARGETOS-$TARGETARCH/nebula-cert /nebula-cert
 
 VOLUME ["/config"]