aci: sort contents, and pull manifest to the front

If the manifest is the first file then tools do not need to download
the whole image file in order to detect changes or updates.

Sorted contents of ACIs allow for easier comparison, and usage of
tools such as zsync, and deduplication on the server. The price,
sorting by 'tar', is cheap.

Parameter 'f' for 'tar' must be followed by the filename for recent
versions of 'tar'. Mind the order!

closes blablacar#210

aci-builder/build.sh

@@ -42,7 +42,7 @@ if ! [[ -x ${dir}/files/dgr/usr/bin/tar ]]; then
   rm -r ${WORKDIR}
 fi
 
-sudo tar xf ${dir}/rootfs.tar.xz -C ${rootfs}/dgr/
+sudo tar -C ${rootfs}/dgr/ -xf ${dir}/rootfs.tar.xz
 sudo cp -R ${dir}/files/. ${rootfs}
 sudo chown root: ${rootfs}
 cp ${dir}/manifest.json ${target}/manifest
@@ -61,7 +61,8 @@ sudo rm -Rf ${rootfs}/dgr/usr/sbin/
 sudo bash -c "cd ${rootfs}/dgr/usr && ln -s bin sbin && cd -"
 
 cd ${target}
-sudo tar cpfz ../bindata/aci-builder.aci rootfs manifest
+sudo tar --sort=name --numeric-owner -cpzf ../bindata/aci-builder.aci manifest rootfs \
+|| sudo tar -cpzf ../bindata/aci-builder.aci manifest rootfs
 sudo chown ${USER}: ../bindata/aci-builder.aci
 sudo rm -Rf rootfs/
 cd -

aci-tester/build.sh

@@ -31,5 +31,6 @@ curl --fail --silent --show-error --location --remote-time --compressed --create
 chmod +x ${rootfs}/dgr/usr/bin/*
 
 cd ${target}
-tar cpfz ../bindata/aci-tester.aci rootfs manifest
+tar --sort=name --numeric-owner -cpzf ../bindata/aci-tester.aci manifest rootfs \
+|| tar -cpzf ../bindata/aci-tester.aci manifest rootfs
 cd -

dgr/common/tar.go

@@ -1,11 +1,7 @@
 package common
 
 func Tar(destination string, source ...string) error { // remove zip
-	source = append(source, "")
-	source = append(source, "")
-	copy(source[2:], source[0:])
-	source[0] = "cpf"
-	source[1] = destination
-
-	return ExecCmd("tar", source...)
+	params := []string{"--sort=name", "--numeric-owner", "-cpf", destination}
+	params = append(params, source...)
+	return ExecCmd("tar", params...)
 }