update shellcheck v0.7.0

full diff: https://github.com/koalaman/shellcheck/compare/v0.6.0...v0.7.0

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

dockerfiles/Dockerfile.shellcheck

@@ -1,4 +1,4 @@
-FROM    koalaman/shellcheck-alpine:v0.6.0
+FROM    koalaman/shellcheck-alpine:v0.7.0
 RUN     apk add --no-cache bash make
 WORKDIR /go/src/github.com/docker/cli
 ENV     DISABLE_WARN_OUTSIDE_CONTAINER=1