Revert "Build static binaries for Docker, so that we can use 'scratch…

…' image (cs3org#1766)"

This partially reverts commit 07adb3f as there are a few issues
when trying to run revad e.g. go-sqlite3 requires it:

2021-06-15 15:39:31.314 ERR go/src/github/cs3org/reva/cmd/revad/runtime/runtime.go:197 > error starting the http server error="http service dataprovider could not be started,: localfs: error initializing db: localfs: error preparing statement: Binary was compiled with 'CGO_ENABLED=0', go-sqlite3 requires cgo to work. This is a stub" pid=1

Makefile

@@ -102,7 +102,7 @@ ci: build-ci test  lint-ci
 
 # to be run in Docker build
 build-revad-docker: off
-	env CGO_ENABLED=0 go build -ldflags ${BUILD_FLAGS} -o ./cmd/revad/revad ./cmd/revad
+	go build -ldflags ${BUILD_FLAGS} -o ./cmd/revad/revad ./cmd/revad
 build-reva-docker: off
 	env CGO_ENABLED=0 go build -ldflags ${BUILD_FLAGS} -o ./cmd/reva/reva ./cmd/reva
 clean: