fix: use custom BASE_IMAGE arg to override cross-rs Docker base images

Cross 0.2.5 always overrides CROSS_BASE_IMAGE with its own :0.2.5
tagged image. Use a custom BASE_IMAGE build arg instead, which cross
doesn't override, to use the :main tagged images (Ubuntu 20.04+).

This provides modern toolchains (protoc 3.x, libclang 10+) while
keeping the cross 0.2.5 binary compatible with rustc 1.85.0.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Mossaka

Cross.toml

@@ -3,16 +3,16 @@ default-target = "x86_64-unknown-linux-musl"
 
 [target.aarch64-unknown-linux-musl.dockerfile]
 file = "cross/Dockerfile.musl"
-build-args = { CROSS_BASE_IMAGE = "ghcr.io/cross-rs/aarch64-unknown-linux-musl:main", CROSS_CMAKE_SYSTEM_PROCESSOR = "aarch64", CROSS_SYSROOT = "/usr/local/aarch64-linux-musl" }
+build-args = { BASE_IMAGE = "ghcr.io/cross-rs/aarch64-unknown-linux-musl:main", CROSS_CMAKE_SYSTEM_PROCESSOR = "aarch64", CROSS_SYSROOT = "/usr/local/aarch64-linux-musl" }
 
 [target.x86_64-unknown-linux-musl.dockerfile]
 file = "cross/Dockerfile.musl"
-build-args = { CROSS_BASE_IMAGE = "ghcr.io/cross-rs/x86_64-unknown-linux-musl:main", CROSS_CMAKE_SYSTEM_PROCESSOR = "x86_64", CROSS_SYSROOT = "/usr/local/x86_64-linux-musl" }
+build-args = { BASE_IMAGE = "ghcr.io/cross-rs/x86_64-unknown-linux-musl:main", CROSS_CMAKE_SYSTEM_PROCESSOR = "x86_64", CROSS_SYSROOT = "/usr/local/x86_64-linux-musl" }
 
 [target.aarch64-unknown-linux-gnu.dockerfile]
 file = "cross/Dockerfile.gnu"
-build-args = { CROSS_BASE_IMAGE = "ghcr.io/cross-rs/aarch64-unknown-linux-gnu:main" }
+build-args = { BASE_IMAGE = "ghcr.io/cross-rs/aarch64-unknown-linux-gnu:main" }
 
 [target.x86_64-unknown-linux-gnu.dockerfile]
 file = "cross/Dockerfile.gnu"
-build-args = { CROSS_BASE_IMAGE = "ghcr.io/cross-rs/x86_64-unknown-linux-gnu:main" }
+build-args = { BASE_IMAGE = "ghcr.io/cross-rs/x86_64-unknown-linux-gnu:main" }

cross/Dockerfile.gnu

@@ -1,6 +1,6 @@
-ARG CROSS_BASE_IMAGE
+ARG BASE_IMAGE
 ARG CROSS_DEB_ARCH
-FROM $CROSS_BASE_IMAGE
+FROM ${BASE_IMAGE}
 
 ARG CROSS_DEB_ARCH
 RUN dpkg --add-architecture ${CROSS_DEB_ARCH} && \

cross/Dockerfile.musl

@@ -1,5 +1,5 @@
-ARG CROSS_BASE_IMAGE
-FROM $CROSS_BASE_IMAGE
+ARG BASE_IMAGE
+FROM ${BASE_IMAGE}
 
 ARG CROSS_CMAKE_SYSTEM_PROCESSOR
 ARG CROSS_SYSROOT