Publish arm image too (#21)

base image (https://hub.docker.com/_/docker/tags?page=1&name=20.10-dind) do support amd64 and arm64

so we can do too

Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/21

Dockerfile.multiarch

@@ -0,0 +1,31 @@
+FROM --platform=$BUILDPLATFORM golang:1.18 as build
+
+COPY . /src
+WORKDIR /src
+
+ARG TARGETOS TARGETARCH
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg \
+    make build
+
+FROM docker:20.10-dind
+
+ARG BUILDX_VERSION
+
+# renovate: datasource=github-releases depName=docker/buildx
+ENV BUILDX_VERSION="${BUILDX_VERSION:-v0.9.1}"
+
+ENV DOCKER_HOST=unix:///var/run/docker.sock
+
+RUN \
+    apk --update add --virtual .build-deps curl && \
+    mkdir -p /usr/lib/docker/cli-plugins/ && \
+    curl -SsL -o /usr/lib/docker/cli-plugins/docker-buildx "https://github.com/docker/buildx/releases/download/v${BUILDX_VERSION##v}/buildx-v${BUILDX_VERSION##v}.linux-amd64" && \
+    chmod 755 /usr/lib/docker/cli-plugins/docker-buildx && \
+    apk del .build-deps && \
+    rm -rf /var/cache/apk/* && \
+    rm -rf /tmp/*
+
+COPY --from=build /src/docker-buildx /bin/docker-buildx
+
+ENTRYPOINT ["/usr/local/bin/dockerd-entrypoint.sh", "docker-buildx"]