[20.10 backport] CentOS 7: use go-proxy to work around old git version

[thaJeztah]

backport of #635 for the 20.10 branch. Not strictly needed currently, but just in case go modules acts up again 🤣

CentOS 7 comes with a very old (1.8) version of git, which is not compatible
with go modules. We previously applied this fix to individual build scripts
for rootless-extras and the scan-cli-plugin, but now that other bits are
failing as well, lets move this to the Dockerfile for this distro, so that
for other distros we can use "direct" and fetch from GitHub.

Without this, the build of docker/cli (master branch) failed with:

+ ./scripts/vendor init
+ go mod edit -modfile=vendor.mod -require=github.com/cpuguy83/go-md2man/v2@v2.0.1
+ cp man/tools.go .
+ ./scripts/vendor update
+ go mod tidy -modfile=vendor.mod
go: github.com/theupdateframework/notary@v0.7.1-0.20210315103452-bf96a202a09a requires
    github.com/docker/go@v1.5.1-1.0.20160303222718-d30aec9fd63c: invalid pseudo-version: git fetch --unshallow -f origin in /go/pkg/mod/cache/vcs/48fbd2dfabec81f4c93170677bfc89087d4bec07a2d08f6ca5ce3d17962677ee: exit status 128:
    fatal: git fetch-pack: expected shallow list
make: *** [manpages] Error 1
error: Bad exit status from /var/tmp/rpm-tmp.aKncVr (%build)