feat: set version from git tag at build time

[vlnst]

Right now the version in config/version.go has to be bumped by hand before each release.

This passes the version to the Docker build instead:

• The workflow sets the VERSION build arg to the tag name (or the commit SHA for PR builds).
• The Dockerfile sets config.Version with -ldflags (leading v stripped, so v1.14.0 becomes 1.14.0).
• Local builds without the arg show dev.

Pushing a vX.Y.Z tag is now enough to release.