update workdir to be STRING_OR_LABEL

It kinda doesn't exist in macro-land, so build a hacky string parsing heuristic

examples/assertion/BUILD.bazel

@@ -190,7 +190,7 @@ oci_image(
     volumes = ["/srv/data"],
     # user & workdir
     user = "root",
-    workdir = ["/root"],
+    workdir = "/root",
     # labels & annotations
     labels = {
         "org.opencontainers.image.version": "0.0.0",
@@ -620,7 +620,7 @@ build_test(
     ],
 )
 
-# Case 21: accept workdir param as a target (list input tested in case8)
+# Case 21: accept workdir param as a target label (string input is tested in case8)
 genrule(
     name = "case21_workdir_target",
     outs = ["case21_workdir_target.txt"],
@@ -630,7 +630,7 @@ genrule(
 oci_image(
     name = "case21",
     base = ":case8",
-    workdir = ":case21_workdir_target",
+    workdir = "case21_workdir_target",
 )
 
 assert_oci_config(

oci/defs.bzl

@@ -33,6 +33,38 @@ def _write_nl_seperated_file(name, kind, elems, forwarded_kwargs):
     )
     return label
 
+
+# There's no "is this a label or just a string?" check in macro land, so
+# approximate it with some silly string heuristics. See
+# https://bazel.build/concepts/labels for label name rules
+def _is_a_workdir_label(input):
+    if input[0] in (":", "@") or input[:2] in ("@@", "//") or '/' not in input:
+        return True
+    return False
+
+
+def test_is_a_workdir_label():
+    testdata = {
+        # Corner case: "foo" could be either a string or a target. This
+        # implementation chooses target. If you want a relative workdir "foo"
+        # use "./foo"
+        "foo": True,
+        "//foo": True,
+        "@@foo//bar": True,
+        ":foo": True,
+        # These are all not labels
+        "/foo": False,
+        "./foo": False,
+        "foo/bar": False,
+        "../foo": False,
+    }
+
+    for input, expected in testdata.items():
+        value = _is_a_workdir_label(input)
+        if value != expected:
+            fail("_is_a_workdir_label(%s) returned %s, expected %s" % (input, value, expected))
+
+
 def oci_image(name, labels = None, annotations = None, env = None, cmd = None, entrypoint = None, workdir = None, exposed_ports = None, volumes = None, **kwargs):
     """Macro wrapper around [oci_image_rule](#oci_image_rule).
 
@@ -50,18 +82,22 @@ def oci_image(name, labels = None, annotations = None, env = None, cmd = None, e
     deterministic) information when running with `--stamp` flag.  See the example in
     [/examples/labels/BUILD.bazel](https://github.com/bazel-contrib/rules_oci/blob/main/examples/labels/BUILD.bazel).
 
-    **LIST_OR_LABEL**: `cmd`, `entrypoint`, `workdir`, `exposed_ports`, `volumes`
+    **LIST_OR_LABEL**: `cmd`, `entrypoint`, `exposed_ports`, `volumes`
 
     Can be a list of strings, or a file with newlines separating entries.
 
+    **STRING_OR_LABEL**: `workdir`
+
+    A string, or a target text file whose output contains a single line
+
     Args:
         name: name of resulting oci_image_rule
         labels: `DICT_OR_LABEL` Labels for the image config.
         annotations: `DICT_OR_LABEL` Annotations for the image config.
         env: `DICT_OR_LABEL` Environment variables provisioned by default to the running container.
         cmd: `LIST_OR_LABEL` Command & argument configured by default in the running container.
         entrypoint: `LIST_OR_LABEL` Entrypoint configured by default in the running container.
-        workdir: `LIST_OR_LABEL` Workdir configured by default in the running container. Only 1 list entry allowed.
+        workdir: `STRING_OR_LABEL` Workdir configured by default in the running container.
         exposed_ports: `LIST_OR_LABEL` Exposed ports in the running container.
         volumes: `LIST_OR_LABEL` Volumes for the container.
         **kwargs: other named arguments to [oci_image_rule](#oci_image_rule) and
@@ -115,15 +151,13 @@ def oci_image(name, labels = None, annotations = None, env = None, cmd = None, e
             forwarded_kwargs = forwarded_kwargs,
         )
 
-    if types.is_list(workdir):
-        if len(workdir) > 1:
-            fail("workdir MUST only include 1 list element")
-
+    # Support a string for convenience. Create a label on the fly.
+    if workdir != None and not _is_a_workdir_label(workdir):
         workdir_label = "_{}_write_workdir".format(name)
         write_file(
             name = workdir_label,
             out = "_{}.workdir.txt".format(name),
-            content = workdir,
+            content = [workdir],
             **forwarded_kwargs
         )
         workdir = workdir_label

oci/tests/BUILD.bazel

@@ -2,6 +2,7 @@ load("@aspect_bazel_lib//lib:diff_test.bzl", "diff_test")
 load("@aspect_bazel_lib//lib:run_binary.bzl", "run_binary")
 load("@bazel_skylib//rules:build_test.bzl", "build_test")
 load(":pull_tests.bzl", "parse_image_test")
+load("//oci:defs.bzl", "test_is_a_workdir_label")
 
 IMAGES_TO_TEST = {
     "linux/amd64": {
@@ -83,3 +84,5 @@ build_test(
 )
 
 parse_image_test(name = "parse_image_test")
+
+test_is_a_workdir_label()