Support host configuration for docker-daemon sources and destinations

This PR adds CLI support for overriding the default docker daemon host when using the
`docker-daemon` transport.

Additionally, to support daemons listening on plain HTTP only, it is possible to
specify the scheme as part of the host URL, e.g. `http+tcp://localhost:2375`

I'm not sure if this is a good idea - I needed a way to stop the httpClient from
trying to use TLS, and this seemed like a reasonable approach, similar to how the
python docker client behaves. Happy to discuss and iterate on this.

I'm guessing that I'd need to sumbit that part upstream to `containers/image` anyway.

Fixes containers#244

cmd/skopeo/copy.go

@@ -160,5 +160,15 @@ var copyCmd = cli.Command{
 			Name:  "dest-compress",
 			Usage: "Compress tarball image layers when saving to directory using the 'dir' transport. (default is same compression type as source)",
 		},
+		cli.StringFlag{
+			Name:  "src-daemon-host",
+			Value: "",
+			Usage: "use docker daemon host at `HOST` (docker-daemon sources only)",
+		},
+		cli.StringFlag{
+			Name:  "dest-daemon-host",
+			Value: "",
+			Usage: "use docker daemon host at `HOST` (docker-daemon destinations only)",
+		},
 	},
 }

cmd/skopeo/utils.go

@@ -22,6 +22,9 @@ func contextFromGlobalOptions(c *cli.Context, flagPrefix string) (*types.SystemC
 		OCISharedBlobDirPath:        c.String(flagPrefix + "shared-blob-dir"),
 		DirForceCompress:            c.Bool(flagPrefix + "compress"),
 		AuthFilePath:                c.String("authfile"),
+		DockerDaemonHost:            c.String(flagPrefix + "daemon-host"),
+		DockerDaemonCertPath:        c.String(flagPrefix + "cert-dir"),
+		DockerDaemonInsecureSkipTLSVerify: !c.BoolT(flagPrefix + "tls-verify"),
 	}
 	if c.IsSet(flagPrefix + "tls-verify") {
 		ctx.DockerInsecureSkipTLSVerify = !c.BoolT(flagPrefix + "tls-verify")