build: Use proton sdk docker image for building.

docker/Makefile

@@ -2,7 +2,7 @@ STEAMRT_VERSION = 0.20210126.1
 STEAMRT_URLBASE = registry.gitlab.steamos.cloud
 
 PROTONSDK_URLBASE = $(STEAMRT_URLBASE)/proton/soldier/sdk
-PROTONSDK_VERSION = $(STEAMRT_VERSION)-0
+PROTONSDK_VERSION = $(STEAMRT_VERSION)-0-dev
 
 # this is just for building toolchain, as we do static builds it should
 # not have any impact on the end result, but changing it will invalidate

docker/README.md

@@ -0,0 +1,81 @@
+Proton (soldier) SDK
+====================
+
+These are the build rules that are used to create docker images to build
+Proton. The automated creation of the official images lives in
+<https://gitlab.steamos.cloud/proton/soldier/sdk>, but this can be
+used to create local images too.
+
+Local usage
+-----------
+
+The `protonsdk_version` make variable will override which image is used
+to build Proton, but may not trigger a full rebuild, so building from
+scratch may be necessary.
+
+Building Proton with a locally build docker image, instead of using the
+official images, can be done by using `protonsdk_version=local`. This
+may be used for instance to test changes to the docker image recipes.
+
+Or, it is also possible to build the docker images first by invoking
+`make protonsdk` and it will tag the images with the `protonsdk_version`
+variable value.
+
+Official images
+---------------
+
+To update the official Proton SDK images:
+
+1) Update the image build rules, `STEAMRT_VERSION` and
+   `PROTONSDK_VERSION` version numbers in this folder, test locally,
+   commit and push the changes.
+
+2) Update `.gitlab-ci.yml` in the
+   [Proton SDK](https://gitlab.steamos.cloud/proton/soldier/sdk)
+   repository to point to the new commit, commit and push to trigger a
+   new build of "-dev" images.
+
+3) Once the images are satifying, tag the version in Proton SDK
+   repository and push the tag, this will trigger a new build of the
+   images and version them with the same tag as the Git tag.
+
+4) Once the images have been published, update the default
+   `protonsdk_version` version number in `Makefile` to use the newly
+   built images by default.
+
+Any change or addition to GPL-ed source first requires to update or add
+the corresponding source to <https://repo.steampowered.com/proton-sdk>.
+The `SOURCES_URLBASE` variable must be used to download the sources
+from there, and its sha256 must be added to validate the sources in the
+same way the existing code does.
+
+Technical details
+-----------------
+
+The images are built incrementally, with intermediate images created
+for each component of the toolchain, then assembled together in a single
+`proton` image.
+
+The reason behind this is to optimize the use of docker cache, so that
+components that didn't change do not require to be rebuilt every time,
+saving a lot of time in the build process, when only small additions are
+made.
+
+It also lets us build several components of the toolchain separately,
+in parallel. This is especially the case when building the images on
+the <https://gitlab.steamos.cloud> CI.
+
+Because the `ARG` parameter in Dockerfile doesn't behave nicely with
+docker caches it is avoided, and `Dockerfile.in` are used with variable
+replacements done using `sed`.
+
+The <https://gitlab.steamos.cloud> CI uses Kaniko instead of Docker,
+with a bit of script conversion to generate commands usable there. More
+details are available in
+[Proton SDK](https://gitlab.steamos.cloud/proton/soldier/sdk).
+
+The `build-base` images are there to create a common ground to build the
+other elements of the toolchain. They are based on fairly recent (more
+than what `steamrt` provides), but they are only used temporarily to
+build a static version of the Binutils, MinGW and GCC binaries, which
+are then copied over the `steamrt` base image.