build(docker): optimize Dockerfile with smaller image and cleanup

[leonamvasquez]

This PR improves the Dockerfile used to build the K9s binary:

• Removes unnecessary dependencies such as vim
• Adds OCI-compliant labels for image metadata
• Disables CGO and applies -trimpath for reproducible builds
• Cleans up build dependencies to reduce final image size
• Replaces redundant platform flag based on Docker BuildKit warning

These improvements make the image more production-ready, smaller, and secure.

[BryanDollery]

Isn't it simpler/more-efficient to just import the kubectl binary from another image? If you do this:

FROM bitnami/kubectl:1.29 as kubectl_image

before line 23, the final FROM statement, then you can add:

COPY --chmod=0755 --from=kubectl_image /opt/bitnami/kubectl/bin/kubectl /usr/local/bin/kubectl

Which seems cleaner than install and uninstalling curl just so that you can download kubectl.

Alternatively, you could use the ADD command:

ADD --chmod=0755 "https://dl.k8s.io/release/${KUBECTL_VERSION}/bin/linux/${TARGETARCH}/kubectl" /usr/local/bin/kubectl

Which is closer to your original intent, but the target arch is taken care of automatically with the FROM version, I believe, but the version is a little more abstracted.

[goya]

or use wget which is there anyway.