Build the Docker driver for arm64

**What this PR does / why we need it**:
Add ARM64 build and release of the Docker driver in Drone pipeline

**Which issue(s) this PR fixes**:
Fixes grafana#5682

**Special notes for your reviewer**:
I would have loved to have a unified x64 and arm64 build but apparently Docker drivers does not support multi arch images.
So instead I went with tweaking the build steps to allow cross building the image for ARM64 and added the instructions to do so in `drone.yml`.

**Checklist**
- [x] Reviewed the [`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md) guide (**required**)
- [x] Documentation added
- [ ] Tests updated
- [ ] Changes that require user attention or interaction to upgrade are documented in `docs/sources/upgrading/_index.md`

Makefile

@@ -458,13 +458,17 @@ endif
 LOKI_DOCKER_DRIVER ?= "grafana/loki-docker-driver"
 PLUGIN_TAG ?= $(IMAGE_TAG)
 PLUGIN_ARCH ?=
+PLUGIN_BUILD_ARGS ?=
+ifeq ("$(PLUGIN_ARCH)", "-arm64")
+	PLUGIN_BUILD_ARGS = --build-arg GOARCH=arm64
+endif
 
 # build-rootfs
 # builds the plugin rootfs
 define build-rootfs
 	rm -rf clients/cmd/docker-driver/rootfs || true
 	mkdir clients/cmd/docker-driver/rootfs
-	docker build --build-arg $(BUILD_IMAGE) -t rootfsimage -f clients/cmd/docker-driver/Dockerfile .
+	docker build $(PLUGIN_BUILD_ARGS) --build-arg $(BUILD_IMAGE) -t rootfsimage -f clients/cmd/docker-driver/Dockerfile .
 
 	ID=$$(docker create rootfsimage true) && \
 	(docker export $$ID | tar -x -C clients/cmd/docker-driver/rootfs) && \
@@ -481,7 +485,7 @@ docker-driver: docker-driver-clean ## build the docker-driver executable
 	docker plugin create $(LOKI_DOCKER_DRIVER):main$(PLUGIN_ARCH) clients/cmd/docker-driver
 
 clients/cmd/docker-driver/docker-driver:
-	CGO_ENABLED=0 go build $(GO_FLAGS) -o $@ ./$(@D)
+	CGO_ENABLED=0 GOARCH=$(GOARCH) go build $(GO_FLAGS) -o $@ ./$(@D)
 
 docker-driver-push: docker-driver
 ifndef DOCKER_PASSWORD

clients/cmd/docker-driver/Dockerfile

@@ -1,16 +1,29 @@
 ARG BUILD_IMAGE=grafana/loki-build-image:0.34.0
+ARG GOARCH=amd64
 # Directories in this file are referenced from the root of the project not this folder
 # This file is intended to be called from the root like so:
-# docker build -t grafana/loki -f cmd/loki/Dockerfile .
+# docker build -t grafana/loki-docker-driver -f clients/cmd/docker-driver/Dockerfile .
 
-# TODO: add cross-platform support
 FROM $BUILD_IMAGE AS build
 COPY . /src/loki
 WORKDIR /src/loki
-RUN make clean && make BUILD_IN_CONTAINER=false clients/cmd/docker-driver/docker-driver
 
-FROM alpine:3.20.3
-RUN apk add --update --no-cache ca-certificates tzdata
+ARG GOARCH
+RUN make clean && make BUILD_IN_CONTAINER=false GOARCH=${GOARCH} clients/cmd/docker-driver/docker-driver
+
+FROM alpine:3.20.3 AS temp
+
+ARG GOARCH
+
+RUN apk add --update --no-cache --arch=${GOARCH} ca-certificates tzdata
+
+FROM --platform=linux/${GOARCH} alpine:3.20.3
+
+COPY --from=temp /etc/ca-certificates.conf /etc/ca-certificates.conf
+COPY --from=temp /usr/share/ca-certificates /usr/share/ca-certificates
+COPY --from=temp /usr/share/zoneinfo /usr/share/zoneinfo
+
 COPY --from=build /src/loki/clients/cmd/docker-driver/docker-driver /bin/docker-driver
+
 WORKDIR /bin/
 ENTRYPOINT [ "/bin/docker-driver" ]

docs/sources/send-data/docker-driver/_index.md

@@ -31,6 +31,9 @@ Run the following command to install the plugin, updating the release version if
 ```bash
 docker plugin install grafana/loki-docker-driver:2.9.2 --alias loki --grant-all-permissions
 ```
+{{% admonition type="note" %}}
+Add `-arm64` to the image tag for AMR64 hosts.
+{{% /admonition %}}
 
 To check installed plugins, use the `docker plugin ls` command. 
 Plugins that have started successfully are listed as enabled: