CheckIfDockerChecksumWasUpdated.sh

#!/bin/bash

touch /tmp/Recent.xyz

while true
do
    echo "Checking again..."
    wget -q https://apt.dockerproject.org/repo/dists/ubuntu-xenial/main/binary-amd64/Release -O /tmp/Release.xyz
    if test /tmp/Release.xyz -nt Recent.xyz; then
        mplayer /usr/share/sounds/ubuntu/stereo/phone-incoming-call.ogg
    fi
    sleep 5;

done

Isn't InRelease.xyz always going to be newer than Recent.xyz here on the very first run? I ran your script, and it just played the sound immediately.

Normally, wget retains the timestamp of the file from the URL. If you have configured your locale properly on your Linux system, then it should show the original time (it is the one at 9 minutes past the hour), converted to reflect your own timezone.

To verify that the timezone is configured properly on your Ubuntu box, run

 dpkg-reconfigure tzdata

(it will show your current timezone, and allow to change it, if you wish so).

(the update has taken place, so Docker can again be installed properly from the Docker repositories for Debian and Ubuntu).

There is weird issue with the directory at https://apt.dockerproject.org/repo/dists/ubuntu-xenial/main/binary-amd64/
If you notice, there is no file called "InRelease" shown over there.

However, if you

$ wget -S --no-cache https://apt.dockerproject.org/repo/dists/ubuntu-xenial/main/binary-amd64/InRelease
--2016-06-02 17:27:28-- https://apt.dockerproject.org/repo/dists/ubuntu-xenial/main/binary-amd64/InRelease
Resolving apt.dockerproject.org (apt.dockerproject.org)... 52.85.19.64
Connecting to apt.dockerproject.org (apt.dockerproject.org)|52.85.19.64|:443... connected.
HTTP request sent, awaiting response...
HTTP/1.1 200 OK
Content-Type: text/plain
Content-Length: 2576
Connection: keep-alive
Date: Thu, 02 Jun 2016 13:32:44 GMT
x-amz-meta-s3cmd-attrs: uid:0/gname:root/uname:root/gid:0/mode:33188/mtime:1464865616/atime:1464826124/md5:124af13d48d3b02cea1d01d4596e73fd/ctime:1464865616
Last-Modified: Thu, 02 Jun 2016 11:09:23 GMT
x-amz-version-id: kM1R5Po2.wkPK5Ic5hT.4J_QqlmIdA3g
ETag: "124af13d48d3b02cea1d01d4596e73fd"
Server: AmazonS3
Age: 211
X-Cache: Hit from cloudfront
Via: 1.1 e5d27f3fb83b3b7e4d92ffc70e7b5a1f.cloudfront.net (CloudFront)
X-Amz-Cf-Id: W5jsFWNhTMZlkuWUqg1cwSb-7x0gYmopmAog1gMDsiI9vJlM7Tt5FQ==
Length: 2576 (2,5K) [text/plain]
Saving to: ‘InRelease’

InRelease 100%[==========================>] 2,52K --.-KB/s in 0s

2016-06-02 17:27:28 (454 MB/s) - ‘InRelease’ saved [2576/2576]

That is, Cloudfront serves a cached file, the one that used to show up earlier with the wrong checksums.
In other words, although the file InRelease is gone, Cloudfront still serves it.