fix(ci): loki-canary cross platform

Makefile

@@ -372,6 +372,8 @@ loki-push:
 # loki-canary
 loki-canary-image:
 	$(SUDO) $(BUILD_OCI) -t $(IMAGE_PREFIX)/loki-canary:$(IMAGE_TAG) -f cmd/loki-canary/Dockerfile .
+loki-canary-push:
+	$(SUDO) $(PUSH_OCI) $(IMAGE_PREFIX)/loki-canary:$(IMAGE_TAG)
 
 # build-image (only amd64)
 build-image: OCI_PLATFORMS=

cmd/loki-canary/Dockerfile

@@ -1,14 +1,17 @@
 # Directories in this file are referenced from the root of the project not this folder
 # This file is intented to be called from the root like so:
 # docker build -t grafana/promtail -f cmd/promtail/Dockerfile .
+FROM golang:1.11.4-alpine as goenv
+RUN go env GOARCH > /goarch && \
+  go env GOARM > /goarm
 
-FROM grafana/loki-build-image:0.2.1 as build
-ARG GOARCH="amd64"
+FROM --platform=linux/amd64 grafana/loki-build-image as build
+COPY --from=goenv /goarch /goarm /
 COPY . /go/src/github.com/grafana/loki
 WORKDIR /go/src/github.com/grafana/loki
-RUN make clean && make loki-canary
+RUN GOARCH=$(cat /goarch) GOARM=$(cat /goarm) make clean && make loki-canary
 
-FROM       alpine:3.9
-RUN        apk add --update --no-cache ca-certificates
-COPY       --from=build /go/src/github.com/grafana/loki/cmd/loki-canary/loki-canary /usr/bin/loki-canary
+FROM alpine:3.9
+RUN apk add --update --no-cache ca-certificates
+COPY --from=build /go/src/github.com/grafana/loki/cmd/loki-canary/loki-canary /usr/bin/loki-canary
 ENTRYPOINT [ "/usr/bin/loki-canary" ]