refactor: Override options redesign

* Add test for defaults, fix platform detection

* Improve correctness of reader.identifier with block

* Fix options test to be multiplatform

* 'Refactored by Sourcery'

docs: write a section on overrides

tests: test docker launches

feat: add identifiers to launches

test: add test for correct build step generation

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

unit_test/linux_build_steps_test.py

@@ -0,0 +1,70 @@
+import textwrap
+from pathlib import Path
+from pprint import pprint
+
+import cibuildwheel.docker_container
+import cibuildwheel.linux
+from cibuildwheel.options import Options
+
+from .utils import get_default_command_line_arguments
+
+
+def test_linux_container_split(tmp_path: Path, monkeypatch):
+    """
+    Tests splitting linux builds by docker image and before_all
+    """
+
+    args = get_default_command_line_arguments()
+    args.platform = "linux"
+
+    (tmp_path / "pyproject.toml").write_text(
+        textwrap.dedent(
+            """
+                [tool.cibuildwheel]
+                manylinux-x86_64-image = "normal_docker_image"
+                manylinux-i686-image = "normal_docker_image"
+                build = "*-manylinux_x86_64"
+                skip = "pp*"
+                archs = "x86_64 i686"
+
+                [[tool.cibuildwheel.overrides]]
+                select = "cp{38,39,310}-*"
+                manylinux-x86_64-image = "other_docker_image"
+                manylinux-i686-image = "other_docker_image"
+
+                [[tool.cibuildwheel.overrides]]
+                select = "cp39-*"
+                before-all = "echo 'a cp39-only command'"
+            """
+        )
+    )
+
+    monkeypatch.chdir(tmp_path)
+    options = Options("linux", command_line_arguments=args)
+
+    python_configurations = cibuildwheel.linux.get_python_configurations(
+        options.globals.build_selector, options.globals.architectures
+    )
+
+    build_steps = list(cibuildwheel.linux.get_build_steps(options, python_configurations))
+
+    # helper functions to extract test info
+    def identifiers(step):
+        return [c.identifier for c in step.platform_configs]
+
+    def before_alls(step):
+        return [options.build_options(c.identifier).before_all for c in step.platform_configs]
+
+    pprint(build_steps)
+
+    assert build_steps[0].docker_image == "normal_docker_image"
+    assert identifiers(build_steps[0]) == ["cp36-manylinux_x86_64", "cp37-manylinux_x86_64"]
+    assert before_alls(build_steps[0]) == ["", ""]
+
+    assert build_steps[1].docker_image == "other_docker_image"
+    assert identifiers(build_steps[1]) == ["cp38-manylinux_x86_64", "cp310-manylinux_x86_64"]
+    assert before_alls(build_steps[1]) == ["", ""]
+
+    assert build_steps[2].docker_image == "other_docker_image"
+    assert identifiers(build_steps[2]) == ["cp39-manylinux_x86_64"]
+    assert before_alls(build_steps[2]) == ["echo 'a cp39-only command'"]