Add alpine3.6 variants

[tianon]

FYI @dhiltgen @dmp42 @ncopa

(This will get us s390x/golang+Alpine support -- they're built from the same sources as-is.)

[tianon]

See #131 (comment) (and linked bits) for why we aren't updating the alpine variants directly and are instead adding new variants.

[tianon]

$ git diff --no-index 1.7/alpine 1.7/alpine3.6
diff --git a/1.7/alpine/Dockerfile b/1.7/alpine3.6/Dockerfile
index bba761f..dfebca4 100644
--- a/1.7/alpine/Dockerfile
+++ b/1.7/alpine3.6/Dockerfile
@@ -1,4 +1,4 @@
-FROM alpine:3.4
+FROM alpine:3.6
 
 RUN apk add --no-cache ca-certificates
 

$ git diff --no-index 1.8/alpine 1.8/alpine3.6
diff --git a/1.8/alpine/Dockerfile b/1.8/alpine3.6/Dockerfile
index 7a0d7fd..04fa35f 100644
--- a/1.8/alpine/Dockerfile
+++ b/1.8/alpine3.6/Dockerfile
@@ -1,4 +1,4 @@
-FROM alpine:3.5
+FROM alpine:3.6
 
 RUN apk add --no-cache ca-certificates