The `lstrip` and `rstrip` functions take a set of characters to remove, not a
prefix/suffix. Thus `rstrip('-x86_64')` will remove any trailing characters in
the string `'-x86_64'` in any order (in effect it strips the suffix matching
the regex `[-_x468]*`). So with `18.09.4` it removes the `4` suffix resulting
in trying to `int('')` later on:

    Traceback (most recent call last):
      File "/src/scripts/versions.py", line 80, in <module>
        main()
      File "/src/scripts/versions.py", line 73, in main
        versions, reverse=True, key=operator.attrgetter('order')
      File "/src/scripts/versions.py", line 52, in order
        return (int(self.major), int(self.minor), int(self.patch)) + stage
    ValueError: invalid literal for int() with base 10: ''

Since we no longer need to check for the arch suffix (since it no longer
appears in the URLs we are traversing) we could just drop the `rstrip` and
invent a local prefix stripping helper to replace `lstrip('docker-')`. Instead
lets take advantage of the behaviour of `re.findall` which is that if the regex
contains a single `()` match that will be returned. This lets us match exactly
the sub-section of the regex we require.

While editing the regex, also ensure that the suffix is precisely `.tgz` and
not merely `tgz` by adding an explicit `\.`, previously the literal `.` would
be swallowed by the `.*` instead.
Signed-off-by: Ian Campbell <ijc@docker.com>