Add (make test-skopeo) and run it in Travis CI

[mtrmac]

Note that this is branched against commit 1a7569b , not latest master; latest master currently breaks skopeo (pending containers/skopeo#133 ).

Fixes #13 .

[mtrmac]

(First Travis run, let’s see if this works at all…)

[mtrmac]

I am ambivalent about the default value. Personally SUDO = sudo would make my life easier, but the empty string default is less surprising, and on systems which do not need root privileges to use Docker this does not gain unnecessary privileges and is not unexpectedly interactive.

[runcom]

Either way works for me

[mtrmac]

… huh, Travis tests the branch against latest master? Anyway, close enough to working for now, will rebase as needed.

[runcom]

So are we waiting on #23 and containers/skopeo#133 to merge this eventually?

[mtrmac]

Yes. I guess we could do without #23 but the blocker is containers/skopeo#133 anyway IIUC.

[mtrmac]

👍 Finally, both are in sync and this works.

@runcom , can we please merge this before changing anything else about containers/image and before the two get out of sync again?

[runcom]

lgtm