Trouble installing nginx-plus on Alpine docker image

Greetings,

I have an NGINX+ free trial and I want to get an NGINX+ container up and running for my bosses before we purchase a paid subscription.

So I'm following the instructions for installing NGINX+ on Alpine Linux and creating a docker NGINX+ image based on Alpine:3.10. The docker build command fails at line 29 (see attached Dockerfile).

apk add nginx-plus

All the previous commands succeed. The error reads...

ERROR: https://plus-pkgs.nginx.com/alpine/v3.10/main: Permission denied
WARNING: Ignoring APKINDEX.affd029a.tar.gz: No such file or directory
ERROR: unsatisfiable constraints:
nginx-plus (missing):
required by: world[nginx-plus]

The build command looks like so...

PS> $version = "1.0"
PS> $proxyUrl = "http://" + $username + ":" + $password + "@our-company-proxy:11111"
PS> docker build --tag nginxplus:$version --build-arg http_proxy=$proxyUrl --build-arg https_proxy=$proxyUrl --no-cache .

Does anyone know how to fix this error?

Thanks!

Carl T.