build: prevent -dirty tag in Docker builds

Ensure Docker builds don't add a `-dirty` suffix to the version number.

When building for Docker, `make` is called which in turn calls `git describe` to get the version number. Sometimes this would append `-dirty` even when clean.

This change adds `git update-index -q --refresh` before calling `make` to ensure the version tag is returned correctly.

Dockerfile

@@ -2,6 +2,7 @@
 FROM golang:latest AS build-stage
 WORKDIR /build
 COPY . .
+RUN git update-index -q --refresh
 RUN make
 
 FROM alpine:latest