docker: ensure the hubble binary is statially linked

From the CGO documentation[0]:

> The cgo tool is enabled by default for native builds on systems where
> it is expected to work. It is disabled by default when
> cross-compiling.  You can control this by setting the CGO_ENABLED
> environment variable when running the go tool: set it to 1 to enable
> the use of cgo, and to 0 to disable it.

This commits enforces disabling CGO to ensure that the hubble binary is
statically linked.

[0]: https://golang.org/cmd/cgo/

Signed-off-by: Robin Hahling <[email protected]>

Author: rolinh

Dockerfile

@@ -2,7 +2,7 @@ FROM docker.io/library/golang:1.14.3-alpine3.11 as builder
 WORKDIR /go/src/github.com/cilium/hubble
 RUN apk add --no-cache git make
 COPY . .
-RUN make clean && make hubble
+RUN make clean && CGO_ENABLED=0 make hubble
 
 FROM docker.io/library/alpine:3.11
 RUN apk add --no-cache bash curl jq