Bump grafana/promtail from 2.7.4 to 3.1.2 in /docker

Bumps grafana/promtail from 2.7.4 to 3.1.2.

---
updated-dependencies:
- dependency-name: grafana/promtail
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

docker/Dockerfile

@@ -8,7 +8,7 @@ RUN go mod download
 COPY . .
 RUN CGO_ENABLED=0 GOOS=linux GOARCH=$GOARCH go build -o /loki-cf-logdrain
 
-FROM grafana/promtail:2.7.4 AS promtail
+FROM grafana/promtail:3.1.2 AS promtail
 
 ## Build final image
 FROM debian:buster