upd(build): clean up build step, use crust-builder

.drone.yml

 workspace:
-  base: /go
-  path: src/github.com/crusttech/crust
+  base: /go/src
+  path: github.com/crusttech/crust
 
 kind: pipeline
 name: crust
 
 steps:
 - name: build
-  image: golang:1.11-alpine
+  image: crusttech/crust-builder:latest
+  pull: always
   environment:
     CGO_ENABLED: 0
     GOOS: linux
     GOARCH: amd64
   commands:
-  - apk add --no-cache git make
-  - go fmt ./cmd/... ./internal/... ./crm/... ./messaging/... ./system/...
-  - go get -u github.com/golang/mock/gomock
-  - go get -u github.com/golang/mock/mockgen
-  - make mocks
   - go build -ldflags "-X github.com/crusttech/crust/internal/version.BuildTime=`date +%FT%T%z` -X github.com/crusttech/crust/internal/version.Version=`git describe --always --tags`" -o build/crust-$GOOS-$GOARCH cmd/crust/*.go
   - go build -ldflags "-X github.com/crusttech/crust/internal/version.BuildTime=`date +%FT%T%z` -X github.com/crusttech/crust/internal/version.Version=`git describe --always --tags`" -o build/crm-$GOOS-$GOARCH cmd/crm/*.go
   - go build -ldflags "-X github.com/crusttech/crust/internal/version.BuildTime=`date +%FT%T%z` -X github.com/crusttech/crust/internal/version.Version=`git describe --always --tags`" -o build/messaging-$GOOS-$GOARCH cmd/messaging/*.go