refactor: rename new attribute on oci_tarball (#453)

oci/private/tarball.bzl

@@ -39,9 +39,17 @@ attrs = {
         allow_single_file = [".txt"],
         mandatory = True,
     ),
-    "command": attr.label(
+    "loader": attr.label(
         doc = """\
-            target for a container cli tool (i.e. docker or podman or other) that will be used to load the image into the local engine when using 'bazel run //my/image'.",
+            Alternative target for a container cli tool that will be
+            used to load the image into the local engine when using `bazel run` on this oci_tarball.
+
+            By default, we look for `docker` or `podman` on the PATH, and run the `load` command.
+            
+            > Note that rules_docker has an "incremental loader" which has better performance, see
+            > Follow https://github.com/bazel-contrib/rules_oci/issues/454 for similar behavior in rules_oci.
+
+            See the _run_template attribute for the script that calls this loader tool.
             """,
         allow_single_file = True,
         mandatory = False,
@@ -51,7 +59,10 @@ attrs = {
     "_run_template": attr.label(
         default = Label("//oci/private:tarball_run.sh.tpl"),
         doc = """ \
-              The template used to load the container. The default template uses Docker, but this template could be replaced to use podman, runc, or another runtime. Please reference the default template to see available substitutions. 
+              The template used to load the container when using `bazel run` on this oci_tarball.
+              
+              See the `loader` attribute to replace the tool which is called.
+              Please reference the default template to see available substitutions. 
         """,
         allow_single_file = True,
     ),
@@ -99,13 +110,13 @@ def _tarball_impl(ctx):
         output = exe,
         substitutions = {
             "{{image_path}}": tarball.short_path,
-            "{{command}}": ctx.file.command.path if ctx.file.command else "",
+            "{{loader}}": ctx.file.loader.path if ctx.file.loader else "",
         },
         is_executable = True,
     )
     runfiles = [tarball]
-    if ctx.file.command:
-        runfiles.append(ctx.file.command)
+    if ctx.file.loader:
+        runfiles.append(ctx.file.loader)
 
     return [
         DefaultInfo(files = depset([tarball]), runfiles = ctx.runfiles(files = runfiles), executable = exe),

oci/private/tarball_run.sh.tpl

@@ -2,15 +2,15 @@
 set -o pipefail -o errexit -o nounset
 
 readonly IMAGE="{{image_path}}"
-if [ -e "{{command}}" ]; then
-    CONTAINER_CLI="{{command}}"
+if [ -e "{{loader}}" ]; then
+    CONTAINER_CLI="{{loader}}"
 elif command -v docker &> /dev/null; then
     CONTAINER_CLI="docker"
 elif command -v podman &> /dev/null; then
     CONTAINER_CLI="podman"
 else
     echo >&2 "Neither docker or podman could be found."
-    echo >&2 "If you wish to use another container runtime, you can pass an executable to the 'command' attribute."
+    echo >&2 "To use a different container runtime, pass an executable to the 'loader' attribute of oci_tarball."
     exit 1
 fi