Bump grafana/mimirtool from 3.0.0 to 3.0.3 in /mimir (#8)

Bumps grafana/mimirtool from 3.0.0 to 3.0.3.

---
updated-dependencies:
- dependency-name: grafana/mimirtool
  dependency-version: 3.0.3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

mimir/Dockerfile

@@ -1,5 +1,5 @@
 FROM busybox:uclibc as builder
-FROM grafana/mimirtool:3.0.0 as mimirtool
+FROM grafana/mimirtool:3.0.3 as mimirtool
 FROM grafana/mimir:3.0.0
 COPY --from=builder /bin/sh /bin/sh
 COPY --from=builder /bin/busybox /bin/busybox