Exec shell: autodetect container shell

Some containers may have bash installed but some have only basic sh (Ash/Dash).
There is other shells too: ZSh, tsh etc.

We can automatically detect what shell is configured.
See explanation how it works moby/moby#41702

docs/Config.md

@@ -47,7 +47,7 @@ customCommands:
   containers:
   - name: bash
     attach: true
-    command: docker exec -it {{ .Container.ID }} /bin/sh
+    command: "docker exec -it {{ .Container.ID }} /bin/sh -c 'eval $(grep ^$(id -un): /etc/passwd | cut -d : -f 7-)'"
     serviceNames: []
 oS:
   openCommand: open {{filename}}

pkg/config/app_config.go

@@ -344,7 +344,7 @@ func GetDefaultConfig() UserConfig {
 			Containers: []CustomCommand{
 				{
 					Name:    "bash",
-					Command: "docker exec -it {{ .Container.ID }} /bin/sh",
+					Command: "docker exec -it {{ .Container.ID }} /bin/sh -c 'eval $(grep ^$(id -un): /etc/passwd | cut -d : -f 7-)'",
 					Attach:  true,
 				},
 			},