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>

docs/options.md

@@ -107,6 +107,56 @@ The complete set of defaults for the current version of cibuildwheel are shown b
     not want to change a `pyproject.toml` file. You can specify a different file to
     use with `--config-file` on the command line, as well.
 
+### Configuration overrides {: #overrides }
+
+One feature specific to the configuration files is the ability to override
+settings based on selectors. To use, add a ``tool.cibuildwheel.overrides``
+array, and specify a ``select`` string. Then any options you set will only
+apply to items that match that selector. These are applied in order, with later
+matches overriding earlier ones if multiple selectors match. Environment
+variables always override static configuration.
+
+A few of the options below have special handling in overrides. A different
+`before-all` will trigger a new docker launch on Linux, and cannot be
+overridden on macOS or Windows.  Overriding the image on linux will also
+generate new docker launches, one per image.  Some commands are not supported;
+`output-dir`, build/skip/test_skip selectors, and architectures cannot be
+overridden.
+
+##### Examples:
+
+```toml
+[tool.cibuildwheel.linux]
+before-all = "yum install mylib"
+test-command = "echo 'installed'"
+
+[[tool.cibuildwheel.overrides]]
+select = "*-musllinux*"
+before-all = "apk add mylib"
+```
+
+This example will override the before-all command on musllinux only, but will
+still run the test-command. Note the double brackets, this is an array in TOML,
+which means it can be given multiple times.
+
+```toml
+[tool.cibuildwheel]
+# Normal options, etc.
+manylinux-x86_64-image = "manylinux2010"
+
+[[tool.cibuildwheel.overrides]]
+select = "cp36-*"
+manylinux-x86_64-image = "manylinux1"
+
+[[tool.cibuildwheel.overrides]]
+select = "cp310-*"
+manylinux-x86_64-image = "manylinux2014"
+```
+
+This example will build CPython 3.6 wheels on manylinux1, CPython 3.7-3.9
+images on manylinux2010, and CPython 3.10 wheels on manylinux2014.
+
+
 ## Options summary
 
 <div class="options-toc"></div>
@@ -305,7 +355,8 @@ If not listed above, `auto` is the same as `native`.
 Platform-specific environment variables are also available:<br/>
  `CIBW_ARCHS_MACOS` | `CIBW_ARCHS_WINDOWS` | `CIBW_ARCHS_LINUX`
 
-This option can also be set using the [command-line option](#command-line) `--archs`.
+This option can also be set using the [command-line option](#command-line)
+`--archs`. This option cannot be set in an `overrides` section in `pyproject.toml`.
 
 #### Examples
 
@@ -564,6 +615,10 @@ The placeholder `{package}` can be used here; it will be replaced by the path to
 
 On Windows and macOS, the version of Python available inside `CIBW_BEFORE_ALL` is whatever is available on the host machine. On Linux, a modern Python version is available on PATH.
 
+This option has special behavior in the overrides section in `pyproject.toml`.
+On linux, overriding it triggers a new docker launch. It cannot be overridden
+on macOS and Windows.
+
 Platform-specific environment variables also available:<br/>
 `CIBW_BEFORE_ALL_MACOS` | `CIBW_BEFORE_ALL_WINDOWS` | `CIBW_BEFORE_ALL_LINUX`
 
@@ -1144,6 +1199,8 @@ This will skip testing on any identifiers that match the given skip patterns (se
 
 With macOS `universal2` wheels, you can also skip the individual archs inside the wheel using an `:arch` suffix. For example, `cp39-macosx_universal2:x86_64` or `cp39-macosx_universal2:arm64`.
 
+This option is not supported in the overrides section in `pyproject.toml`.
+
 #### Examples
 
 !!! tab examples "Environment variables"