improve install from source

* Idempotent clone_and_build.sh does everything
* Add documentation for how to build in Docker

Had to sacrificy go generate because stringer apparently can't handle
vendor directory used by go dep, fails with error
on go generate rpc/frame_layer.go

refs #37

Makefile

@@ -7,14 +7,14 @@ _TESTPKGS := $(ROOT) $(foreach p,$(SUBPKGS),$(ROOT)/$(p))
 
 ARTIFACTDIR := artifacts
 
-build: generate
-		go build -o $(ARTIFACTDIR)/zrepl
-
-generate:
+generate: #not part of the build, must do that manually
 	@for pkg in $(_TESTPKGS); do\
 		go generate "$$pkg" || exit 1; \
 	done;
 
+build:
+		go build -o $(ARTIFACTDIR)/zrepl
+
 test:
 	@for pkg in $(_TESTPKGS); do \
 		echo "Testing $$pkg"; \
@@ -39,7 +39,7 @@ cover: artifacts
 artifacts:
 	mkdir artifacts
 
-release: generate artifacts vet test
+release: artifacts vet test
 	GOOS=linux GOARCH=amd64   go build -o "$(ARTIFACTDIR)/zrepl-linux-amd64"
 	GOOS=freebsd GOARCH=amd64 go build -o "$(ARTIFACTDIR)/zrepl-freebsd-amd64"