Ensure prefix check for path traversal has path separator

[wagoodman]

Reported by @a1loy , it is possible to specify a tar header that matches the random destination path as a prefix, but additionally has more elements as a suffix. In these cases we will still write out to a stereoscope temp directory that is cleaned up (thus this is not a security concern), but could allow for unintentional writes within that temp directory. This addresses the bug by explicitly performing a prefix check with a path separator.

[github-actions]

Benchmark Test Results

Benchmark results from the latest changes vs base branch

latest: Pulling from library/ubuntu
tar: Option --mtime: Treating date 'UTC 2019-09-16' as 2019-09-16 00:00:00
goos: linux
goarch: amd64
pkg: github.com/anchore/stereoscope/pkg/file
cpu: AMD EPYC 7763 64-Core Processor                
ctr: 
           │ ./.tmp/benchmark-2e4783f.txt │
           │            sec/op            │
TarIndex-2                   35.02µ ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

           │ ./.tmp/benchmark-2e4783f.txt │
           │             B/op             │
TarIndex-2                  5.560Ki ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

           │ ./.tmp/benchmark-2e4783f.txt │
           │          allocs/op           │
TarIndex-2                    93.00 ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

pkg: github.com/anchore/stereoscope/test/integration
                                      │ ./.tmp/benchmark-2e4783f.txt │
                                      │            sec/op            │
SimpleImage_GetImage/docker-archive-2                   1.276m ± ∞ ¹
SimpleImage_GetImage/podman-2                           18.67m ± ∞ ¹
geomean                                                 4.881m
¹ need >= 6 samples for confidence interval at level 0.95

                                      │ ./.tmp/benchmark-2e4783f.txt │
                                      │             B/op             │
SimpleImage_GetImage/docker-archive-2                  329.7Ki ± ∞ ¹
SimpleImage_GetImage/podman-2                          435.1Ki ± ∞ ¹
geomean                                                378.7Ki
¹ need >= 6 samples for confidence interval at level 0.95

                                      │ ./.tmp/benchmark-2e4783f.txt │
                                      │          allocs/op           │
SimpleImage_GetImage/docker-archive-2                   2.789k ± ∞ ¹
SimpleImage_GetImage/podman-2                           2.753k ± ∞ ¹
geomean                                                 2.771k
¹ need >= 6 samples for confidence interval at level 0.95

ctr: failed to dial "/run/containerd/containerd.sock": connection error: desc = "transport: error while dialing: dial unix /run/containerd/containerd.sock: connect: permission denied"
                                                   │ ./.tmp/benchmark-2e4783f.txt │
                                                   │            sec/op            │
SimpleImage_FetchSquashedContents/docker-archive-2                   18.13µ ± ∞ ¹
SimpleImage_FetchSquashedContents/podman-2                           18.17µ ± ∞ ¹
geomean                                                              18.15µ
¹ need >= 6 samples for confidence interval at level 0.95

                                                   │ ./.tmp/benchmark-2e4783f.txt │
                                                   │             B/op             │
SimpleImage_FetchSquashedContents/docker-archive-2                  2.648Ki ± ∞ ¹
SimpleImage_FetchSquashedContents/podman-2                          2.648Ki ± ∞ ¹
geomean                                                             2.648Ki
¹ need >= 6 samples for confidence interval at level 0.95

                                                   │ ./.tmp/benchmark-2e4783f.txt │
                                                   │          allocs/op           │
SimpleImage_FetchSquashedContents/docker-archive-2                    21.00 ± ∞ ¹
SimpleImage_FetchSquashedContents/podman-2                            21.00 ± ∞ ¹
geomean                                                               21.00
¹ need >= 6 samples for confidence interval at level 0.95