Dockerfiles: Use cross compilation for kubectl-gadget Dockerfile.

[eiffel-fl]

Hi.

This PR makes kubectl-gadget.Dockerfile using cross compilation, it speeds up build for other architectures:

$ docker buildx build --load --platform=linux/arm64 -f Dockerfiles/kubectl-gadget.Dockerfile -t kubectl-gadget .
[+] Building 235.5s (14/14) FINISHED                                                                     docker-container:magical_sammet
 => [internal] load build definition from kubectl-gadget.Dockerfile                                                                 0.0s
 => => transferring dockerfile: 1.89kB                                                                                              0.0s
 => [internal] load .dockerignore                                                                                                   0.0s
 => => transferring context: 2B                                                                                                     0.0s
 => [internal] load metadata for docker.io/library/alpine:3.18                                                                      2.1s
 => [internal] load metadata for docker.io/library/golang:1.20-bullseye                                                             2.0s
...
 => [builder 5/5] RUN cd /gadget && GOHOSTOS=linux GOHOSTARCH=arm64 make kubectl-gadget                                            41.4s
...
 => importing to docker
$ docker run --rm -ti kubectl-gadget            francis/kubectl-gadget-docker-cross *%
WARNING: The requested image's platform (linux/arm64) does not match the detected host platform (linux/amd64/v3) and no specific platform was requested
/ #
# Other terminal
$ docker cp angry_shockley:/bin/kubectl-gadget /tmp 
Successfully copied 87.2MB to /tmp
$ file /tmp/kubectl-gadget                      francis/kubectl-gadget-docker-cross *%
/tmp/kubectl-gadget: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, Go BuildID=jowbtesoByV0av3PcG9O/KQcXh1JQmFyFXFmi6AAs/yFwYPe_b5K_Poppn2F1f/kevBJNNvk740Te9iA4OM, with debug_info, not stripped

Best regards.

[burak-ok]

Thanks for configuring it that fast.
LGTM

[eiffel-fl]

Thank you for the review!