fix: report actual version instead of hardcoded 3.0.0-dev

- Add version variable to main.go, set via -X ldflags at build time
- Pass version through to Server, used in the / endpoint response
- Makefile passes VERSION from git describe
- Dockerfile accepts VERSION build arg
- Woodpecker passes CI_COMMIT_TAG as VERSION

.woodpecker/docker.yaml

@@ -8,6 +8,8 @@ steps:
     settings:
       registry: git.unkin.net
       repo: git.unkin.net/unkin/artifactapi
+      build_args:
+        VERSION: ${CI_COMMIT_TAG}
       username: droneci
       password:
         from_secret: DRONECI_PASSWORD