Use official shellcheck image

This patch switches the shellcheck image to use the official image
from Docker Hub. In order to keep the image minimal, the validate/shellcheck
script is called directly (so that `make` is not needed), and is now using
`/bin/sh` instead of `bash` (which wasn't needed to perform the checks).

Note that this does not yet update shellcheck to the latest version, because
checks fail with the v0.5 version. Instead, the latest v0.4.x version is used,
which is roughly the same version as was installed in the image before this
change;

```
docker run --rm docker-cli-shell-validate shellcheck --version
ShellCheck - shell script analysis tool
version: 0.4.4
license: GNU General Public License, version 3
website: http://www.shellcheck.net
```

Signed-off-by: Sebastiaan van Stijn <[email protected]>

docker.Makefile

@@ -107,7 +107,7 @@ yamldocs: build_docker_image ## generate documentation YAML files consumed by do
 
 .PHONY: shellcheck
 shellcheck: build_shell_validate_image ## run shellcheck validation
-	docker run -ti --rm $(ENVVARS) $(MOUNTS) $(VALIDATE_IMAGE_NAME) make shellcheck
+	docker run -ti --rm $(ENVVARS) $(MOUNTS) $(VALIDATE_IMAGE_NAME) scripts/validate/shellcheck
 
 .PHONY: test-e2e ## run e2e tests
 test-e2e: test-e2e-non-experimental test-e2e-experimental test-e2e-connhelper-ssh

dockerfiles/Dockerfile.shellcheck

@@ -1,10 +1,2 @@
-FROM    debian:stretch-slim
-
-RUN     apt-get update && \
-        apt-get -y install make shellcheck && \
-        apt-get clean
-
+FROM    koalaman/shellcheck-alpine:v0.4.7
 WORKDIR /go/src/github.com/docker/cli
-ENV     DISABLE_WARN_OUTSIDE_CONTAINER=1
-CMD     bash
-COPY    . .

scripts/validate/shellcheck

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 set -eo pipefail
 
 shellcheck contrib/completion/bash/docker