Use default http transport settings to honor proxy settings

[wagoodman]

Before #174 the http.Transport was only overridden in certain circumstances. When the transport was overridden no proxy configuration would be honored. With the containerd PR, the transport is always overridden, causing proxy configurations to fail by default. This fixes the overridden proxy to always inherit all options (except for the TLS config) from the default prototype to fix the existing proxy issues reported in anchore/syft#2203 .

[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-46b87f1.txt │
           │            sec/op            │
TarIndex-2                   35.31µ ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

           │ ./.tmp/benchmark-46b87f1.txt │
           │             B/op             │
TarIndex-2                  5.559Ki ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

           │ ./.tmp/benchmark-46b87f1.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-46b87f1.txt │
                                      │            sec/op            │
SimpleImage_GetImage/docker-archive-2                   1.303m ± ∞ ¹
SimpleImage_GetImage/podman-2                           19.02m ± ∞ ¹
geomean                                                 4.978m
¹ need >= 6 samples for confidence interval at level 0.95

                                      │ ./.tmp/benchmark-46b87f1.txt │
                                      │             B/op             │
SimpleImage_GetImage/docker-archive-2                  329.3Ki ± ∞ ¹
SimpleImage_GetImage/podman-2                          435.3Ki ± ∞ ¹
geomean                                                378.6Ki
¹ need >= 6 samples for confidence interval at level 0.95

                                      │ ./.tmp/benchmark-46b87f1.txt │
                                      │          allocs/op           │
SimpleImage_GetImage/docker-archive-2                   2.776k ± ∞ ¹
SimpleImage_GetImage/podman-2                           2.755k ± ∞ ¹
geomean                                                 2.765k
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-46b87f1.txt │
                                                   │            sec/op            │
SimpleImage_FetchSquashedContents/docker-archive-2                   17.94µ ± ∞ ¹
SimpleImage_FetchSquashedContents/podman-2                           17.94µ ± ∞ ¹
geomean                                                              17.94µ
¹ need >= 6 samples for confidence interval at level 0.95

                                                   │ ./.tmp/benchmark-46b87f1.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-46b87f1.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