fix https://github.com/Neilpang/acme.sh/issues/1127

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine
+FROM alpine:3.6
 
 RUN apk update -f \
   && apk --no-cache add -f \

acme.sh

@@ -463,8 +463,7 @@ if _exists xargs && [ "$(printf %s '\\x41' | xargs printf)" = 'A' ]; then
 fi
 
 _h2b() {
-  if _exists xxd; then
+  if _exists xxd && xxd -r -p 2>/dev/null; then
-    xxd -r -p
     return
   fi