Set engine env from common config

Set the env that is used by Podman.
related issue containers/common#31

Signed-off-by: Qi Wang <qiwan@redhat.com>

cmd/podman/root.go

@@ -128,6 +128,21 @@ func persistentPreRunE(cmd *cobra.Command, args []string) error {
 		return err
 	}
 
+	for _, env := range cfg.Engine.Env {
+		splitEnv := strings.SplitN(env, "=", 2)
+		if len(splitEnv) != 2 {
+			return fmt.Errorf("invalid environment variable for engine %s, valid configuration is KEY=value pair", env)
+		}
+		// skip if the env is already defined
+		if _, ok := os.LookupEnv(splitEnv[0]); ok {
+			logrus.Debugf("environment variable %s is already defined, skip the settings from containers.conf", splitEnv[0])
+			continue
+		}
+		if err := os.Setenv(splitEnv[0], splitEnv[1]); err != nil {
+			return err
+		}
+	}
+
 	if cmd.Flag("cpu-profile").Changed {
 		f, err := os.Create(cfg.CPUProfile)
 		if err != nil {

docs/source/markdown/podman.1.md

@@ -123,6 +123,10 @@ NOTE --tmpdir is not used for the temporary storage of downloaded images.  Use t
 
 Print the version
 
+## Environment Variables
+
+Podman can set up environment variables from env of [engine] table in containers.conf. These variables can be overridden by passing  environment variables before the `podman` commands.
+
 ## Exit Status
 
 The exit code from `podman` gives information about why the container