Replace bash with sh

Alpine images implement sh instead of bash. Do they fail this guideline, or is it good enough to provide any shell available?

README.md

@@ -71,7 +71,7 @@ No official images can be derived from, or depend on, non-official images with t
 
 #### Consistency
 
-All official images should provide a consistent interface. A beginning user should be able to `docker run official-image bash` without needing to learn about `--entrypoint`. It is also nice for advanced users to take advantage of entrypoint, so that they can `docker run official-image --arg1 --arg2` without having to specify the binary to execute.
+All official images should provide a consistent interface. A beginning user should be able to `docker run official-image sh` without needing to learn about `--entrypoint`. It is also nice for advanced users to take advantage of entrypoint, so that they can `docker run official-image --arg1 --arg2` without having to specify the binary to execute.
 
 1.	If the startup process does not need arguments, just use `CMD`:
 
@@ -86,10 +86,10 @@ All official images should provide a consistent interface. A beginning user shou
 	CMD ["postgres"]
 	```
 
-	1.	Ensure that `docker run official-image bash` works too. The easiest way is to check for the expected command and if it is something else, just `exec "$@"` (run whatever was passed, properly keeping the arguments escaped).
+	1.	Ensure that `docker run official-image sh` works too. The easiest way is to check for the expected command and if it is something else, just `exec "$@"` (run whatever was passed, properly keeping the arguments escaped).
 
-		```bash
-		#!/bin/bash
+		```sh
+		#!/bin/sh
 		set -e
 
 		# this if will check if the first argument is a flag
@@ -106,7 +106,7 @@ All official images should provide a consistent interface. A beginning user shou
 		    exec gosu mongod "$@"
 		fi
 
-		# else default to run whatever the user wanted like "bash"
+		# else default to run whatever the user wanted like "sh"
 		exec "$@"
 		```