set --ssh default when omitted

Signed-off-by: Alex Couture-Beil <alex@earthly.dev>

commands/build.go

@@ -309,7 +309,7 @@ func buildCmd(dockerCli command.Cli, rootOpts *rootOptions) *cobra.Command {
 
 	flags.StringArrayVar(&options.secrets, "secret", []string{}, "Secret file to expose to the build: id=mysecret,src=/local/secret")
 
-	flags.StringArrayVar(&options.ssh, "ssh", []string{}, "SSH agent socket or keys to expose to the build (format: default|<id>[=<socket>|<key>[,<key>]])")
+	flags.StringArrayVar(&options.ssh, "ssh", []string{"default"}, "SSH agent socket or keys to expose to the build (format: default|<id>[=<socket>|<key>[,<key>]])")
 
 	flags.StringArrayVarP(&options.outputs, "output", "o", []string{}, "Output destination (format: type=local,dest=path)")