docker: init fetchdocker nix code for docker2nix

This change adds granular, non-docker daemon docker image fetchers and
a docker image layer compositor to be used in conjunction with the
`docker2nix` utility provided by the `haskellPackages.hocker` package.

This change includes a hackage package version bump and updated sha256
for recent fixes released to `hocker` resulting from formulating this
patch.

pkgs/build-support/fetchdocker/fetchdocker-builder.sh

@@ -0,0 +1,28 @@
+source "${stdenv}/setup"
+header "exporting ${repository}/${imageName} (tag: ${tag}) into ${out}"
+mkdir -p "${out}"
+
+cat <<EOF > "${out}/compositeImage.sh"
+#! ${bash}/bin/bash
+#
+# Create a tar archive of a docker image's layers, docker image config
+# json, manifest.json, and repositories json; this streams directly to
+# stdout and is intended to be used in concert with docker load, i.e:
+# 
+# ${out}/compositeImage.sh | docker load
+
+# The first character follow the 's' command for sed becomes the
+# delimiter sed will use; this makes the transformation regex easy to
+# read. We feed tar a file listing the files we want in the archive,
+# because the paths are absolute and docker load wants them flattened in
+# the archive, we need to transform all of the paths going in by
+# stripping everything *including* the last solidus so that we end up
+# with the basename of the path.
+${gnutar}/bin/tar \
+  --transform='s=.*/==' \
+  --transform="s=.*-manifest.json=manifest.json=" \
+  --transform="s=.*-repositories=repositories=" \
+  -c "${manifest}" "${repositories}" -T "${imageFileStorePaths}"
+EOF
+chmod +x "${out}/compositeImage.sh"
+stopNest