fix(docker): Use the right shebang in healthcheck.sh (#963)

Alpine images don't have bash installed by default, so we need to use
`/bin/sh` instead. This follows the *same existing convention that
we follow in the `entrypoint.sh` script*.

Both ubuntu and alpine images have been tested (i.e healthchecks to
pass) to work with this change.

tools/docker/healthcheck.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 HOST="localhost"
 PORT=6379