build: removes gopkg files from dev docker file (#16817)

Dockerfile

@@ -3,14 +3,10 @@ FROM golang:1.12.4
 
 WORKDIR $GOPATH/src/github.com/grafana/grafana
 
-COPY Gopkg.toml Gopkg.lock ./
+COPY go.mod go.sum ./
 COPY vendor vendor
 
-ARG DEP_ENSURE=""
-RUN if [ ! -z "${DEP_ENSURE}" ]; then \
-      go get -u github.com/golang/dep/cmd/dep && \
-      dep ensure --vendor-only; \
-    fi
+RUN go mod verify
 
 COPY pkg pkg
 COPY build.go build.go

UPGRADING_DEPENDENCIES.md

@@ -18,10 +18,21 @@ Upgrading Go or Node.js requires making changes in many different files. See bel
 
 ## Go Dependencies
 
-Updated using `dep`.
+The Grafana project uses [Go modules](https://golang.org/cmd/go/#hdr-Modules__module_versions__and_more) to manage dependencies on external packages. This requires a working Go environment with version 1.11 or greater installed.
 
-- `Gopkg.toml`
-- `Gopkg.lock`
+All dependencies are vendored in the `vendor/` directory.
+
+To add or update a new dependency, use the `go get` command:
+
+```bash
+# Pick the latest tagged release.
+go get example.com/some/module/pkg
+
+# Pick a specific version.
+go get example.com/some/module/pkg@vX.Y.Z
+```
+
+Tidy up the `go.mod` and `go.sum` files and copy the new/updated dependency to the `vendor/` directory:
 
 ## Node.js Dependencies