cmd-build: Add lockfile support

First, we teach `build` to always output a lockfile, which we store into
the final build dir. Second, we also teach `build` to take a
`--lockfile` argument to enforce a specific lockfile.

This is the first step towards actually being able to make use of
lockfiles in the FCOS pipeline.

src/cmd-build

@@ -24,10 +24,11 @@ EOF
 }
 
 # Parse options
+LOCKFILE=
 FORCE=
 SKIP_PRUNE=0
 rc=0
-options=$(getopt --options hf --longoptions help,force,force-nocache,skip-prune -- "$@") || rc=$?
+options=$(getopt --options hf --longoptions lockfile:,help,force,force-nocache,skip-prune -- "$@") || rc=$?
 [ $rc -eq 0 ] || {
     print_help
     exit 1
@@ -45,6 +46,10 @@ while true; do
         --skip-prune)
             SKIP_PRUNE=1
             ;;
+        --lockfile)
+            shift
+            LOCKFILE=$1
+            ;;
         --)
             shift
             break
@@ -153,9 +158,13 @@ prepare_git_artifacts "${configdir_gitrepo}" "${PWD}/coreos-assembler-config.tar
 
 # These need to be absolute paths right now for rpm-ostree
 composejson=${PWD}/tmp/compose.json
+# Put this under tmprepo so it gets automatically chown'ed if needed
+lockfile_out=${tmprepo}/tmp/lockfile.json
 # --cache-only is here since `fetch` is a separate verb.
 runcompose --cache-only ${FORCE} --add-metadata-from-json "${commitmeta_input_json}" \
-           --write-composejson-to "${composejson}"
+           --write-composejson-to "${composejson}" \
+           --ex-write-lockfile-to "${lockfile_out}" \
+           ${LOCKFILE:+--ex-lockfile ${LOCKFILE}}
 # Very special handling for --write-composejson-to as rpm-ostree doesn't
 # write it if the commit didn't change.
 if [ -f "${changed_stamp}" ]; then
@@ -306,6 +315,9 @@ if [ -n "${ref_is_temp}" ]; then
     mv meta.json{.new,}
 fi
 
+# Move lockfile into build dir
+mv "${lockfile_out}" .
+
 # And add the commit metadata itself, which includes notably the rpmdb pkglist
 # in a format that'd be easy to generate diffs out of for higher level tools
 "${dn}"/commitmeta_to_json "${tmprepo}" "${commit}" > commitmeta.json