Support additional_contexts

Signed-off-by: Otto Liljalaakso <otto.liljalaakso@novatron.fi>

tests/additional_contexts/README.md

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+# Test podman-compose with build.additional_contexts
+
+```
+podman-compose build
+podman-compose up
+podman-compose down
+```
+
+expected output would be
+
+```
+[dict] | Data for dict
+[list] | Data for list
+```

tests/additional_contexts/data_for_dict/data.txt

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+Data for dict

tests/additional_contexts/data_for_list/data.txt

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+Data for list

tests/additional_contexts/project/Dockerfile

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+FROM busybox
+COPY --from=data data.txt /data/data.txt
+CMD ["busybox", "cat", "/data/data.txt"]

tests/additional_contexts/project/docker-compose.yml

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+version: "3.7"
+services:
+  dict:
+    build:
+      context: .
+      additional_contexts:
+        data: ../data_for_dict
+  list:
+    build:
+      context: .
+      additional_contexts:
+        - data=../data_for_list

tests/test_podman_compose_additional_contexts.py

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+# SPDX-License-Identifier: GPL-2.0
+
+
+"""Test how additional contexts are passed to podman."""
+
+import os
+import subprocess
+import unittest
+
+from .test_podman_compose import podman_compose_path
+from .test_podman_compose import test_path
+
+
+def compose_yaml_path():
+    """ "Returns the path to the compose file used for this test module"""
+    return os.path.join(test_path(), "additional_contexts", "project")
+
+
+class TestComposeBuildAdditionalContexts(unittest.TestCase):
+    def test_build_additional_context(self):
+        """podman build should receive additional contexts as --build-context
+
+        See additional_context/project/docker-compose.yaml for context paths
+        """
+        cmd = (
+            "coverage",
+            "run",
+            podman_compose_path(),
+            "--dry-run",
+            "--verbose",
+            "-f",
+            os.path.join(compose_yaml_path(), "docker-compose.yml"),
+            "build",
+        )
+        p = subprocess.run(
+            cmd,
+            stdout=subprocess.PIPE,
+            check=False,
+            stderr=subprocess.STDOUT,
+            text=True,
+        )
+        self.assertEqual(p.returncode, 0)
+        self.assertIn("--build-context=data=../data_for_dict", p.stdout)
+        self.assertIn("--build-context=data=../data_for_list", p.stdout)