Bump golang 1.12.8 (CVE-2019-9512, CVE-2019-9514)

go1.12.8 (released 2019/08/13) includes security fixes to the net/http and net/url packages.
See the Go 1.12.8 milestone on our issue tracker for details:

https://github.com/golang/go/issues?q=milestone%3AGo1.12.8

- net/http: Denial of Service vulnerabilities in the HTTP/2 implementation
  net/http and golang.org/x/net/http2 servers that accept direct connections from untrusted
  clients could be remotely made to allocate an unlimited amount of memory, until the program
  crashes. Servers will now close connections if the send queue accumulates too many control
  messages.
  The issues are CVE-2019-9512 and CVE-2019-9514, and Go issue golang.org/issue/33606.
  Thanks to Jonathan Looney from Netflix for discovering and reporting these issues.
  This is also fixed in version v0.0.0-20190813141303-74dc4d7220e7 of golang.org/x/net/http2.
  net/url: parsing validation issue
- url.Parse would accept URLs with malformed hosts, such that the Host field could have arbitrary
  suffixes that would appear in neither Hostname() nor Port(), allowing authorization bypasses
  in certain applications. Note that URLs with invalid, not numeric ports will now return an error
  from url.Parse.
  The issue is CVE-2019-14809 and Go issue golang.org/issue/29098.
  Thanks to Julian Hector and Nikolai Krein from Cure53, and Adi Cohen (adico.me) for discovering
  and reporting this issue.

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

appveyor.yml

@@ -4,7 +4,7 @@ clone_folder: c:\gopath\src\github.com\docker\cli
 
 environment:
   GOPATH: c:\gopath
-  GOVERSION: 1.12.7
+  GOVERSION: 1.12.8
   DEPVERSION: v0.4.1
 
 install:

dockerfiles/Dockerfile.binary-native

@@ -1,4 +1,4 @@
-ARG GO_VERSION=1.12.7
+ARG GO_VERSION=1.12.8
 
 FROM    golang:${GO_VERSION}-alpine
 

dockerfiles/Dockerfile.cross

@@ -1,4 +1,4 @@
-ARG GO_VERSION=1.12.7
+ARG GO_VERSION=1.12.8
 
 FROM	dockercore/golang-cross:${GO_VERSION}
 ENV     DISABLE_WARN_OUTSIDE_CONTAINER=1

dockerfiles/Dockerfile.dev

@@ -1,4 +1,4 @@
-ARG GO_VERSION=1.12.7
+ARG GO_VERSION=1.12.8
 
 FROM    golang:${GO_VERSION}-alpine
 

dockerfiles/Dockerfile.e2e

@@ -1,4 +1,4 @@
-ARG GO_VERSION=1.12.7
+ARG GO_VERSION=1.12.8
 
 FROM docker/containerd-shim-process:a4d1531 AS containerd-shim-process
 

dockerfiles/Dockerfile.lint

@@ -1,4 +1,4 @@
-ARG GO_VERSION=1.12.7
+ARG GO_VERSION=1.12.8
 
 FROM    golang:${GO_VERSION}-alpine