Use official shellcheck image

This patch switches the shellcheck image to use the official image
from Docker Hub.

Note that this does not yet update shellcheck to the latest version (v0.5.x);
Shellcheck v0.4.7 added some new checks, which makes CI currently fail, so will
be done in a follow-up PR. Instead, the v0.4.6 version is used in this PR, which
is closest to 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 <github@gone.nl>

dockerfiles/Dockerfile.shellcheck

@@ -1,10 +1,7 @@
-FROM    debian:stretch-slim
+FROM    koalaman/shellcheck-alpine:v0.4.6
-
+RUN     apk add --no-cache bash make
-RUN     apt-get update && \
-        apt-get -y install make shellcheck && \
-        apt-get clean
-
 WORKDIR /go/src/github.com/docker/cli
 ENV     DISABLE_WARN_OUTSIDE_CONTAINER=1
-CMD     bash
+ENTRYPOINT [""]
+CMD     ["/bin/sh"]
 COPY    . .

scripts/warn-outside-container

@@ -1,9 +1,9 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 set -eu
 
 target="${1:-}"
 
-if [[ "$target" != "help" && -z "${DISABLE_WARN_OUTSIDE_CONTAINER:-}" ]]; then
+if [ "$target" != "help" ] && [ -z "${DISABLE_WARN_OUTSIDE_CONTAINER:-}" ]; then
     (
         echo
         echo