scripts/version.py: Use regex grouping to extract the version

[ijc]

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

[ijc]

This issue is taking out all PR CI runs at the moment (since 18.09.4 was released).

[ulyssessouza]

LGTM

[shin-]

Holy carp. Sorry about that 😓 Thanks for getting a fix out @ijc !

[ijc]

No worries! The names of those functions are so misleading I'm surprised it doesn't happen more often!