nonsdk_container_setup: correct sources line changing

New version of Ubuntu LXD images seem to have a slightly different
sources line. Account for those by making the trailing slash optional.

Fixes: #71

crossbuilder

@@ -433,7 +433,7 @@ nonsdk_container_setup () {
         i386-*|amd64-*)
             # Host is on archive, target is on ports.
             exec_container_root "sed -E \
-                -e 's:(archive|security)\.ubuntu\.com/ubuntu/:ports.ubuntu.com/ubuntu-ports/:' \
+                -e 's:(archive|security)\.ubuntu\.com/ubuntu/? :ports.ubuntu.com/ubuntu-ports/ :' \
                 -e 's:^deb :deb [arch=${TARGET_ARCH}] :' \
                 /etc/apt/sources.list >/etc/apt/sources.list.d/ports.list"
             exec_container_root "sed -i -E \
@@ -443,8 +443,8 @@ nonsdk_container_setup () {
         *-i386|*-amd64)
             # Host is on ports, target is on archive.
             exec_container_root "sed -E \
-                -e 's:ports\.ubuntu\.com/ubuntu-ports/ ([a-z]+)-security :security.ubuntu.com/ubuntu/ \1-security :' \
-                -e 's:ports\.ubuntu\.com/ubuntu-ports/ ([a-z-]+):archive.ubuntu.com/ubuntu/ \1 :' \
+                -e 's:ports\.ubuntu\.com/ubuntu-ports/? ([a-z]+)-security :security.ubuntu.com/ubuntu \1-security :' \
+                -e 's:ports\.ubuntu\.com/ubuntu-ports/? ([a-z-]+):archive.ubuntu.com/ubuntu \1 :' \
                 -e 's:^deb :deb [arch=${TARGET_ARCH}] :' \
                 /etc/apt/sources.list >/etc/apt/sources.list.d/non-ports.list"
             exec_container_root "sed -i -E \