fix: enhance Dockerfile for multi-platform builds

[thiagoalmeidasa]

Description of the change

This PR improves the support for building multiple platforms Dockerfile.

• Removed explicit platform from base image, using ${BUILDPLATFORM} instead.
• Added TARGETPLATFORM, BUILDPLATFORM, TARGETOS, and TARGETARCH arguments for clarity and flexibility.
• Ensures consistent platform handling throughout the build process.

Benefits

Multi arch builds are working again.

Possible drawbacks

Applicable issues

• fixes Multiarch is not working on container image 3.11 and 3.12 #299

Additional information

Running from my raspberry pi:

$ docker container run -ti --rm  gateway-admision-controller
Loaded image: gateway-admision-controller:latest
{"addr":":8080","app":"gateway-admision-controller","http-server":"webhooks","level":"info","lib":"kubewebhook","msg":"Command config is config.CmdConfig{Debug:false, Development:false, SetGatewayDefault:false, WebhookListenAddr:\":8080\", MetricsListenAddr:\"\", MetricsPath:\"\", TLSCertFilePath:\"\", TLSKeyFilePath:\"\", Gateway:\"\", D
NS:\"\", DNSPolicy:\"\", SetGatewayLabel:\"\", SetGatewayLabelValue:\"\", SetGatewayAnnotation:\"\", SetGatewayAnnotationValue:\"\", InitImage:\"\", InitImagePullPol:\"\", InitCmd:\"\", InitMountPoint:\"\", SidecarImage:\"\", SidecarImagePullPol:\"\", SidecarCmd:\"\", SidecarMountPoint:\"\", SidecarAsInit:false, ConfigmapName:\"\"}","serv
ice":"webhook-handler","time":"2025-05-07T15:11:32Z","version":"dev","webhook":"gatewayPodMutator"}
{"addr":":8080","app":"gateway-admision-controller","http-server":"webhooks","level":"info","lib":"kubewebhook","msg":"Current DNS config is resolv.Resolver{Domains:[]string{}, Nameservers:[]string{\"192.168.100.1\"}, Search:[]string{\"lan\"}, Sortlist:[]string{}, Options:[]resolv.ResolverOption{}}","service":"webhook-handler","time":"202
5-05-07T15:11:32Z","version":"dev","webhook":"gatewayPodMutator"}
{"addr":":8080","app":"gateway-admision-controller","http-server":"webhooks","level":"warning","msg":"webhook running without TLS","time":"2025-05-07T15:11:32Z","version":"dev"}
{"addr":":8080","app":"gateway-admision-controller","http-server":"webhooks","level":"info","msg":"http server listening...","time":"2025-05-07T15:11:32Z","version":"dev"}
^C{"app":"gateway-admision-controller","level":"info","msg":"signal interrupt received","time":"2025-05-07T15:11:33Z","version":"dev"}
{"addr":":8080","app":"gateway-admision-controller","http-server":"webhooks","level":"info","msg":"start draining connections","time":"2025-05-07T15:11:33Z","version":"dev"}
{"addr":":8080","app":"gateway-admision-controller","http-server":"webhooks","level":"info","msg":"server stopped","time":"2025-05-07T15:11:33Z","version":"dev"}

[angelnu]

Thanks for the pr. Merging so we can test the ci.

[DrummyFloyd]

any news about this ? =)