Add shell script linting with shellcheck for better portability

script/binary

@@ -8,9 +8,9 @@ fi
 
 rm -f dist/traefik
 
-FLAGS=""
+FLAGS=()
 if [ -n "$VERBOSE" ]; then
-    FLAGS="${FLAGS} -v"
+    FLAGS+=(-v)
 fi
 
 if [ -z "$VERSION" ]; then
@@ -26,7 +26,7 @@ if [ -z "$DATE" ]; then
 fi
 
 # Build binaries
-CGO_ENABLED=0 GOGC=off go build $FLAGS -ldflags "-s -w \
+CGO_ENABLED=0 GOGC=off go build "${FLAGS[@]}" -ldflags "-s -w \
     -X github.com/containous/traefik/pkg/version.Version=$VERSION \
     -X github.com/containous/traefik/pkg/version.Codename=$CODENAME \
     -X github.com/containous/traefik/pkg/version.BuildDate=$DATE" \