Add CI builder image and use it in the pipeline

builder.Dockerfile bakes the build step's dependencies into
git.teletype.hu/internal/* so the pipeline no longer installs
packages on every run.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

builder.Dockerfile

@@ -0,0 +1,30 @@
+# CI builder image for Bevy projects: Rust toolchain with the
+# wasm32-unknown-unknown target and the wasm-bindgen CLI preinstalled,
+# so the pipeline's build step does not download them on every run.
+#
+# WASM_BINDGEN_VERSION must match the `wasm-bindgen` crate version pinned
+# in the game repos' Cargo.toml (see README) — rebuild and retag this image
+# when bumping it. The CLI is the x86_64 musl binary, so the image must be
+# built for linux/amd64.
+#
+# Build & push (the CI runner is linux/amd64):
+#   docker build --platform linux/amd64 -f builder.Dockerfile \
+#     --build-arg WASM_BINDGEN_VERSION=0.2.100 \
+#     -t git.teletype.hu/internal/bevy-builder:latest .
+#   docker push git.teletype.hu/internal/bevy-builder:latest
+
+FROM rust:1-alpine
+
+ARG WASM_BINDGEN_VERSION=0.2.100
+
+RUN apk add --no-cache musl-dev make zip curl git jq
+
+RUN rustup target add wasm32-unknown-unknown
+
+RUN curl -sSL "https://github.com/rustwasm/wasm-bindgen/releases/download/${WASM_BINDGEN_VERSION}/wasm-bindgen-${WASM_BINDGEN_VERSION}-x86_64-unknown-linux-musl.tar.gz" \
+      | tar xz -C /usr/local/bin --strip-components=1 \
+ && wasm-bindgen --version
+
+WORKDIR /workspace
+
+CMD ["/bin/sh"]