Fix docker buildx install (grafana#10838)

**What this PR does / why we need it**:
I've forgot to copy the binary in
grafana#10833.

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](grafana@d10549e)

.drone/drone.jsonnet

@@ -498,7 +498,7 @@ local manifest_ecr(apps, archs) = pipeline('manifest-ecr') {
 
 [
   pipeline('loki-build-image') {
-    local build_image_tag = '0.31.0',
+    local build_image_tag = '0.31.1',
     workspace: {
       base: '/src',
       path: 'loki',

.drone/drone.yml

@@ -10,7 +10,7 @@ steps:
     dry_run: true
     repo: grafana/loki-build-image
     tags:
-    - 0.31.0
+    - 0.31.1
   when:
     event:
     - pull_request
@@ -26,7 +26,7 @@ steps:
       from_secret: docker_password
     repo: grafana/loki-build-image
     tags:
-    - 0.31.0
+    - 0.31.1
     username:
       from_secret: docker_username
   when:
@@ -2017,6 +2017,6 @@ kind: secret
 name: gpg_private_key
 ---
 kind: signature
-hmac: d7342c3fd4963b8345ff398d8f605f70c04d23cac2da06ad6728d11e1b6671c5
+hmac: 54814c2f4d4645a6cd18d6dcaccffca97131d2a49b35c6efb7a8ad4c44daecc3
 
 ...

loki-build-image/Dockerfile

@@ -96,6 +96,7 @@ RUN dpkg --add-architecture armhf && \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 COPY --from=docker /usr/bin/docker /usr/bin/docker
+COPY --from=docker /usr/libexec/docker/cli-plugins/docker-buildx /usr/libexec/docker/cli-plugins/docker-buildx
 COPY --from=helm /usr/bin/helm /usr/bin/helm
 COPY --from=helm /go/bin/helm-docs /usr/bin/helm-docs
 COPY --from=lychee /usr/bin/lychee /usr/bin/lychee