build: fix docker build on arm/v6

Unexpectedly the team which runs the Go docker images have removed the
arm/v6 image which means that the rclone docker images no longer
build.

One of the recommended fixes is what we've done here - switch to the
alpine builder. This has the advantage that it actually builds arm/v6
architecture unlike the previous builder which build arm/v5.

See: docker-library/golang#502

Dockerfile

@@ -1,8 +1,9 @@
-FROM golang AS builder
+FROM golang:alpine AS builder
 
 COPY . /go/src/github.com/rclone/rclone/
 WORKDIR /go/src/github.com/rclone/rclone/
 
+RUN apk add --no-cache make bash gawk git
 RUN \
   CGO_ENABLED=0 \
   make

RELEASE.md

@@ -124,32 +124,21 @@ Cherry pick any changes back to master and the stable branch if it is active.
 
 ## Making a manual build of docker
 
-The rclone docker image should autobuild on via GitHub actions.  If it doesn't
-or needs to be updated then rebuild like this.
+To do a basic build of rclone's docker image to debug builds locally:
 
-See: https://github.com/ilteoood/docker_buildx/issues/19
-See: https://github.com/ilteoood/docker_buildx/blob/master/scripts/install_buildx.sh
+```
+docker buildx build --load -t rclone/rclone:testing --progress=plain .
+docker run --rm rclone/rclone:testing version
+```
+
+To test the multipatform build
 
 ```
-git co v1.54.1
-docker pull golang
-export DOCKER_CLI_EXPERIMENTAL=enabled
-docker buildx create --name actions_builder --use
-docker run --rm --privileged docker/binfmt:820fdd95a9972a5308930a2bdfb8573dd4447ad3
-docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
-SUPPORTED_PLATFORMS=$(docker buildx inspect --bootstrap | grep 'Platforms:*.*' | cut -d : -f2,3)
-echo "Supported platforms: $SUPPORTED_PLATFORMS"
-docker buildx build --platform linux/amd64,linux/386,linux/arm64,linux/arm/v7 -t rclone/rclone:1.54.1 -t rclone/rclone:1.54 -t rclone/rclone:1 -t rclone/rclone:latest --push .
-docker buildx stop actions_builder
+docker buildx build -t rclone/rclone:testing --progress=plain --platform linux/amd64,linux/386,linux/arm64,linux/arm/v7,linux/arm/v6 .
 ```
 
-### Old build for linux/amd64 only
+To make a full build then set the tags correctly and add `--push`
 
 ```
-docker pull golang
-docker build --rm --ulimit memlock=67108864  -t rclone/rclone:1.52.0 -t rclone/rclone:1.52 -t rclone/rclone:1 -t rclone/rclone:latest .
-docker push rclone/rclone:1.52.0
-docker push rclone/rclone:1.52
-docker push rclone/rclone:1
-docker push rclone/rclone:latest
+docker buildx build --platform linux/amd64,linux/386,linux/arm64,linux/arm/v7 -t rclone/rclone:1.54.1 -t rclone/rclone:1.54 -t rclone/rclone:1 -t rclone/rclone:latest --push .
 ```