Support generating initial lock file when none exists (#169)

Fixes GoogleContainerTools/distroless#1697

Author: gfrankliu

apt/extensions.bzl

@@ -26,7 +26,12 @@ def _distroless_extension(module_ctx):
                     # buildifier: disable=print
                     print("\nNo lockfile was given, please run `bazel run @%s//:lock` to create the lockfile." % install.name)
             else:
-                lockf = lockfile.from_json(module_ctx, module_ctx.read(install.lock))
+                if module_ctx.path(install.lock).exists:
+                    lockf = lockfile.from_json(module_ctx, module_ctx.read(install.lock))
+                else:
+                    # buildifier: disable=print
+                    print("\nSpecified lockfile '%s' not found. An empty lockfile is assumed." % install.lock)
+                    lockf = lockfile.from_json(module_ctx, None)
                 reproducible = True
 
             for (package) in lockf.packages():

apt/private/lockfile.bzl

@@ -51,7 +51,7 @@ def _create(rctx, lock):
         add_package = lambda *args, **kwargs: _add_package(lock, *args, **kwargs),
         add_package_dependency = lambda *args, **kwargs: _add_package_dependency(lock, *args, **kwargs),
         packages = lambda: lock.packages,
-        write = lambda out: rctx.file(out, json.encode_indent(struct(version = lock.version, packages = lock.packages))),
+        write = lambda out: rctx.file(out, json.encode_indent(struct(version = lock.version, packages = lock.packages)), executable = False),
         as_json = lambda: json.encode_indent(struct(version = lock.version, packages = lock.packages)),
     )