Bump docs go version to > 1.14.0. (gravitational#2005)

Without this, `go get` will fail on Linux 5.4.0 (Ubuntu 20.04) when it
hits golang/go#37436, with the following
signature:

  walt@work:~/git/gravity/docs$ make docs
  mkdir -p ../build/docs
  // snip ...
  go: downloading golang.org/x/net v0.0.0-20200707034311-ab3426394381
  runtime: mlock of signal stack failed: 12
  runtime: increase the mlock limit (ulimit -l) or
  runtime: update your kernel to 5.3.15+, 5.4.2+, or 5.5+
  fatal error: mlock failed

While this won't affect our current release infra (kernel 3.10.0), it is
an important fix for developers running affected kernel versions.

docs/Dockerfile

@@ -3,7 +3,7 @@
 # A multi step build is used to keep the go toolchain outside the final container
 
 # milv-builder, contains the whole go toolchain
-FROM quay.io/gravitational/debian-venti:go1.14-stretch AS milv-builder
+FROM quay.io/gravitational/debian-venti:go1.14.4-buster AS milv-builder
 RUN GO111MODULE=on go get -u -v github.com/magicmatatjahu/milv@v0.0.6
 
 # docbox, contains everything for building gravity documentation