Set a default value for SEM_VER build-time variable in Dockerfile (#435)

Also fix a runtime error that happens when the API server's version is not a valid SemVer.

debug.Dockerfile

@@ -28,7 +28,7 @@ RUN go list -f '{{.Path}}@{{.Version}}' -m all | sed 1d | grep -e 'go-cache' -e
 ARG COMMIT_HASH
 ARG GIT_BRANCH
 ARG BUILD_TIMESTAMP
-ARG SEM_VER
+ARG SEM_VER=0.0.0
 
 # Copy and build agent code
 COPY shared ../shared