Makefile: use official alpine images for build

This commit modifies the build tooling so that we use the official
alipne build images rather than the images from the multiarch org. This
requires new logic to parse docker manifests but results in a cleaner
solution with fewer architecture definitions.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

.travis.yml

@@ -13,6 +13,9 @@ go:
 env:
   - GO111MODULE=on DOCKER_CLI_EXPERIMENTAL=enabled
 
+before_install:
+  - sudo apt-get update && sudo apt-get -y install jq
+
 install: true
 
 script: