add auxilary scripts that help managing images

Dockerfile

 FROM alpine:latest
 
 RUN apk add --no-cache docker-cli
+
+COPY scripts/ /usr/local/bin/

README.md

 # docker-cli
 
+Docker CLI tools based on Alpine.
+
+## Helper scripts
+
+### docker-push-image
+
+Tags `<source_image>` as `<target_image>` and pushes it to the registry.
+
+**Usage**:
+
+    docker-push-image <source_image> <target_image>
+
+### docker-push-manifest
+
+Creates and pushes a docker manifest based on multiple source images.
+
+**Usage**:
+
+```
+    docker-push-manifest <manifest> <image_prefix> <tag_suffix> <tag_suffix>..
+
+    <manifest>          The name of the manifest to create and pushed
+    <image_prefix>      The common prefix of each source image
+    <tag_prefix>        One or more suffixes to that are used to specify the
+                        source images
+```

scripts/docker-push-image

+#!/bin/sh
+
+if [ $? -lt 2 ]; then
+    echo "Usage: $0 <source_image> <target_image>"
+    exit 0
+fi
+
+source_image=$1
+target_image=$2
+
+docker tag "$source_image" "$target_image"
+docker push "$target_image"

scripts/docker-push-manifest

+#!/bin/sh
+
+if [ $? -lt 2 ]; then
+    echo "Usage: $0 <manifest> <image_prefix> <tag_suffixes>.."
+    exit 0
+fi
+
+manifest=$1
+image_prefix=$2
+
+shift 2;
+
+if [ $? -lt 1 ]; then
+    echo "No tag suffixes supplied, nothing to do"
+    exit 1
+fi
+
+tag_suffixes="$@"
+
+images=""
+
+for tag_suffix in $tag_suffixes; do
+    image="$image_prefix$tag_suffix"
+    images="$images $image"
+done
+
+echo docker manifest create --amend "$manifest" $images
+docker manifest create --amend "$manifest" $images
+
+echo docker manifest push --purge "$manifest"
+docker manifest push --purge "$manifest"