Set User-Agent string for HTTP requests

[stefanb2]

Is this a BUG REPORT or FEATURE REQUEST? (leave only one on its own line)

/kind feature

(not sure: you could argue this is a regression...)

Description

podman does not set user agent string on HTTP requests. Therefore the default will be used:

User-Agent: Go-http-client/1.1

Steps to reproduce the issue:

1. podman pull host/path:tag
2. check the HTTP server log on the docker registry

Additional information you deem important (e.g. issue happens only occasionally):

I root caused why docker pull repo/path:tag worked with our internal docker repository, but podman pull repo/path:tag didn't. Our setup uses a nginx reverse proxy that only forwards requests to https://repo:443/... if the User-Agent string matches the pattern docker, e.g. docker/1.13. After changing the pattern to (docker|Go-http-client) the problem went away.

But the generic Go-http-client/1.1 isn't really useful for filtering and it is a regression when compared to docker. Some suggestions how to solve this:

1. Fake it, i.e. pretend to be Docker daemon and use something like docker/1.13 (podman).
2. Be honest, i.e. use podman/1.4.4. This would allow people to update they filters properly.
3. one of the above plus a command line option to override it when required?

Output of podman version:

Version:            1.4.4
RemoteAPI Version:  1
Go Version:         go1.12.7
OS/Arch:            linux/amd64