Avoid git cloning from the deb6 image (elastic#6459)

It seems that by now Debian 6 is so old that `git clone` from Github
repositories no longer works. We only needed git clone from it to
get gotpl running, which is a relatively small program so I included it
and its dependency (go-yaml) as vendored files.

Tested `make package` locally.

dev-tools/packer/docker/xgo-image-deb6/.gitignore

@@ -0,0 +1,2 @@
+beats-builder/yaml.v2
+beats-builder/gotpl

dev-tools/packer/docker/xgo-image-deb6/beats-builder/Dockerfile

@@ -20,11 +20,14 @@ RUN ./fetch.sh http://www.tcpdump.org/release/libpcap-1.8.1.tar.gz 32d7526dde8f8
   ./configure --enable-usb=no --enable-bluetooth=no --enable-dbus=no && \
   make
 
-# Old git version which does not support proxy with go get requires to fetch go-yaml directly
-RUN git clone https://github.com/go-yaml/yaml.git /go/src/gopkg.in/yaml.v2
+# Old git version which does not support proxy with go get directly, and git cloning
+# from github also no longer works.
+ADD yaml.v2 /go/src/gopkg.in/yaml.v2
+ADD gotpl /go/src/github.com/tsg/gotpl
 
 # Load gotpl which is needed for creating the templates.
-RUN go get github.com/tsg/gotpl
+RUN cd /go/src/github.com/tsg/gotpl && \
+  go install
 
 # add patch for gopacket
 ADD gopacket_pcap.patch /gopacket_pcap.patch

dev-tools/packer/docker/xgo-image-deb6/build.sh

@@ -1,5 +1,6 @@
 #!/bin/sh
-
+cp -r ../../../vendor/gopkg.in/yaml.v2 beats-builder/yaml.v2
+cp -r ../../../vendor/github.com/tsg/gotpl beats-builder/gotpl
 docker build --rm=true -t tudorg/xgo-deb6-base base/ && \
     docker build --rm=true -t tudorg/xgo-deb6-1.9.4 go-1.9.4/ &&
     docker build --rm=true -t tudorg/beats-builder-deb6 beats-builder