Dockerfile: use gomod to fix build failures

---> Running in 0e7bb4ca4583
+ export 'GOGC=off'
+ export 'GO111MODULE=on'
+ export 'CGO_ENABLED=0'
+ export 'LDFLAGS=-X /config.Version=0.5.0-pre-40-gb8ad012'
+ go build -v '-mod=vendor' -ldflags '-X /config.Version=0.5.0-pre-40-gb8ad012' -o /go/bin/neo-go ./cli/main.go
build command-line-arguments: cannot load github.com/go-redis/redis: open /neo-go/vendor/github.com/go-redis/redis: no such file or directory
The command '/bin/sh -c set -x     && export GOGC=off     && export GO111MODULE=on     && export CGO_ENABLED=0     && export LDFLAGS="-X ${REPO}/config.Version=${VERSION}"     && go build -v -mod=vendor -ldflags "${LDFLAGS}" -o /go/bin/neo-go ./cli/main.go' returned a non-zero code: 1
make: *** [Makefile:19: image] Error 1
Exited with code 2

Dockerfile

@@ -19,6 +19,8 @@ RUN set -x \
     && export GO111MODULE=on \
     && export CGO_ENABLED=0 \
     && export LDFLAGS="-X ${REPO}/config.Version=${VERSION}" \
+    && go mod tidy -v \
+    && go mod vendor \
     && go build -v -mod=vendor -ldflags "${LDFLAGS}" -o /go/bin/neo-go ./cli/main.go
 
 # Executable image