Shrink notify slack container

.gitlab-ci.yml

@@ -161,7 +161,7 @@ include:
 .notify:
   before_script:
-    - apk update && apk add git curl bash
+    - apk add --no-cache curl
   image: "alpine"
   stage: notification_fail

support/notify_slack.sh

-#!/bin/bash
+#!/bin/sh
 # Sends Slack notification ERROR_MSG to CHANNEL
 # An env. variable CI_SLACK_WEBHOOK_URL needs to be set.