Not possible to build with Cloudflare plugin

Resolution of this issue seems incorrectly to be indicated in this closed issue: https://github.com/abiosoft/caddy-docker/issues/195#issue-453099523

when running `docker build -t caddy --build-arg plugins=git,cloudflare,jwt,login,filter,cors,realip,filemanager,cache,expires github.com/abiosoft/caddy-docker.git`, the build process clearly shows:

```
Sending build context to Docker daemon 109.6kB
Step 1/24 : FROM abiosoft/caddy:builder as builder
builder: Pulling from abiosoft/caddy
Digest: sha256:305e08cea6537ff651262d8a9c9fa1676b4f71dee96fa84f91352b3789a9d5b9
Status: Downloaded newer image for abiosoft/caddy:builder
---> 01e4c36185ca
Step 2/24 : ARG version="1.0.3"
---> Using cache
---> 3da066a92b94
Step 3/24 : ARG plugins="git,cors,realip,expires,cache,cloudflare"
---> Using cache
---> a5301b5c145b
```

First issue is that `build` is ignoring the plugins passed to it; why?

The plugins that are installed using the image built above are as follows:

```
me@NAS:/volume1/docker/_wip/caddy$ sudo docker exec -it caddy /bin/sh
/srv # caddy --plugins
Server types:
http

Caddyfile loaders:
short
flag
default

Other plugins:
http.basicauth
http.bind
http.browse
http.errors
http.expvar
http.ext
http.fastcgi
http.gzip
http.header
http.index
http.internal
http.limits
http.log
http.markdown
http.mime
http.pprof
http.proxy
http.push
http.redir
http.request_id
http.rewrite
http.root
http.status
http.templates
http.timeouts
http.websocket
on
tls
tls.cluster.file
```

Second issue: **where is the cloudflare plugin???** even if not accepting the plugins list I passed through to `build`, the cloudflare plugin is in the list that should be installed per the output of `build` so what gives?