Error parsing proxy during fpga-opae docker image build

[skorup101]

Issue

Cannot build docker image for fpga-opae.
It originally happened during installation of OpenNESS. See: https://github.com/open-ness/converged-edge-experience-kits/blob/master/roles/kubernetes/opae_fpga/node/tasks/main.yml#L42

How to reproduce

It happened during ansible OpenNESS installation, but it can be reproduced more easly at https://github.com/open-ness/edgeservices repo level.

1. Clone this repo.
2. Run make fpga-opae.

$ make fpga-opae
...
Step 25/30 : RUN echo "proxy=$http_proxy/" | sudo tee -a /etc/yum.conf
 ---> Running in 24c68f8e724f
proxy=/
Removing intermediate container 24c68f8e724f
 ---> 79bcd801af6f
Step 26/30 : RUN sudo yum clean expire-cache
 ---> Running in 4c1c3d2f758c
Loaded plugins: fastestmirror, ovl
Options error: Error parsing "proxy = '/'": URL must be http, ftp, https, socks4, socks4a, socks5 or socks5h not ""
The command '/bin/sh -c sudo yum clean expire-cache' returned a non-zero code: 1
make: *** [fpga-opae] Error 1

Possible solution

Check if http_proxy variable exists in the Dockerfile.