fixed build.sh image removal

in recent versions of docker the parent image is not saved as a tag when building
to handle both whom has the official golang image saved and whom is using only to build,
now the scripts pulls (and tags) the official image so that the rm command does not fail

build.sh

@@ -7,6 +7,7 @@ set -e
 REGEX="^[ ]*(#.*)?$"
 while read -r VERSION; do
   if [[ ! "${VERSION}" =~ $REGEX ]]; then
+    docker pull golang:${VERSION}
     docker build --force-rm --build-arg GO_VERSION=${VERSION} -t bryanpedini/gobuilder:${VERSION} .
     docker push bryanpedini/gobuilder:${VERSION}
     docker image rm bryanpedini/gobuilder:${VERSION} golang:${VERSION}