Dockerfile: Add missing GOPROXY build argument

Previously, commit 102616b158 tried to
expose GOPROXY env var from developer environment by specifying it as
environment variable or build argument to container build process. But
using --build-arg to container build command would require the same to
be referenced inside Dockerfile/Containerfile so that the following
warning can be avoided:

[Warning] one or more build args were not consumed: [GOPROXY]

Signed-off-by: Anoop C S <anoopcs@cryptolab.net>

testing/containers/ceph/Dockerfile

@@ -24,6 +24,8 @@ ARG GO_VERSION=1.19.5
 ENV GO_VERSION=${GO_VERSION}
 ARG GOARCH
 ENV GOARCH=${GOARCH}
+ARG GOPROXY
+ENV GOPROXY=${GOPROXY}
 RUN true && \
     gotar=go${GO_VERSION}.linux-${GOARCH}.tar.gz && \
     gourl="https://dl.google.com/go/${gotar}" && \