random errors downloading images

[iaguis]

We're seeing some occasional errors fetching images, not sure if they're related to #83 but it might be the case.

Semaphore:

    rkt_tests.go:115: Running command: /home/runner/rkt/builds/build-rkt-coreos/build-rkt-1.7.0+git/tmp/functional/rkt --dir=/tmp/datadir-322458840 --local-config=/tmp/localdir-586074711 --system-config=/tmp/systemdir-459955146 --user-config=/tmp/userdir-702517409 --store-only --insecure-options=image fetch docker://busybox:latest 
    rkt_tests.go:216: Error: ExpectRegex didn't find regex 'sha512-[0-9a-f]{32,64}'.
        Output: image: remote fetching from URL "docker://busybox:latest"

        Downloading sha256:a3ed95caeb0 [===================================] 32 B / 32 B

        fetch: error generating ACI: error writing ACI: unexpected EOF

On fedora-rawhide:

17:48:13    rkt_tests.go:115: Running command: /home/fedora/workspace/rkt-matrix/dummy/dummy1/label/fedora-rawhide-flavor-host/builds/build-rkt-host/build-rkt-1.6.0+git/tmp/functional/rkt --dir=/tmp/datadir-552332049 --local-config=/tmp/localdir-616853308 --system-config=/tmp/systemdir-999719275 --user-config=/tmp/userdir-726059470 --store-only --insecure-options=image fetch docker://busybox
17:48:13     rkt_tests.go:216: Error: ExpectRegex didn't find regex 'sha512-[0-9a-f]{32,64}'.
17:48:13         Output: image: remote fetching from URL "docker://busybox"
17:48:13         Downloading sha256:a3ed95caeb0 [===================================] 32 B / 32 B
17:48:13         fetch: error generating ACI: error writing ACI: flate: corrupt input before offset 45274

[jonboulle]

Another weird example here: https://semaphoreci.com/coreos/rkt/branches/master/builds/1055

[jonboulle]

https://github.com/coreos/pkg/pull/61