Update Docker Client User Agent in CI

[geoand]

We do this so we can get better analytics for the downloaded version of the build images (by being able to easily filter out image downloads originating from CI).

Docker CLI mentioning this capability can be found here.

[geoand]

cc @gsmet @cescoffier

I am going to see what this looks like on my fork before promoting

[geoand]

Before my change the docker config looks like:

{
	"auths": {
		"https://index.docker.io/v1/": {
			"auth": "****"
		}
	}
}

After the change it looks like:

{
  "auths": {
    "https://index.docker.io/v1/": {
      "auth": "****"
    }
  },
  "HttpHeaders": {
    "User-Agent": "Quarkus-CI-Docker-Client"
  }
}

[gastaldi]

LGTM

[gsmet]

Very good idea!

[geoand]

Very good idea!

Thanks :)

[maxandersen]

I would add the github org/repo name into the agent (based on github actons vars) - then we can distinguish between quarkus core builds and contributors.

[geoand]

I thought of that but decided against it because it would potentially make filtering harder (I don't know how filtering is done, so I wanted to be on the safe side)