client: do not fallback to GET if HEAD on _ping fail to connect

When we see an `ECONNREFUSED` (or equivalent) from an attempted `HEAD` on the
`/_ping` endpoint there is no point in trying again with `GET` since the server
is not responding/available at all.

Once vendored into the cli this will partially mitigate docker/cli#1739
("Docker commands take 1 minute to timeout if context endpoint is unreachable")
by cutting the effective timeout in half.

Signed-off-by: Ian Campbell <ijc@docker.com>

client/ping.go

@@ -31,6 +31,8 @@ func (cli *Client) Ping(ctx context.Context) (types.Ping, error) {
 			// Server handled the request, so parse the response
 			return parsePingResponse(cli, serverResp)
 		}
+	} else if IsErrConnectionFailed(err) {
+		return ping, err
 	}
 
 	req, err = cli.buildRequest("GET", path.Join(cli.basePath, "/_ping"), nil, nil)