Docker image: verify sha256 checksum before using remote download

• use locked down alpine version: 3.9 for now, to avoid unexpected updates
• remove glibc: not used by neither of the tools
• remove tools like wget, git from final image: were used as build dependency (git not even used there)
• verify sha256 checksums of remote downloads before using
• use multi layered build for layers cache: for local testing mostly
• actually use kubectl version specified in gitlab-ci variable 💯

image sizes:

• before: 158MB
• after: 98.1MB

there's no info on initial commit: b3b4e409 why there was glibc, or git present, so the above information is based barely on assumptions