Simple dockerfile

Dockerfile

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+# Use `docker build --build-arg RUST_IMAGE_TAG=whatever .` to specify a
+# different variant of the Rust image when building this image.
+ARG RUST_IMAGE_TAG=latest
+
+FROM "docker.io/library/rust:$RUST_IMAGE_TAG"
+
+# Use `docker build --build-arg CARGO_MUTANTS_VERSION=whatever .` to specify the
+# version cargo-mutants to install in this image.
+ARG CARGO_MUTANTS_VERSION
+
+# Install cargo-mutants.
+#
+# - Cache `/usr/local/cargo/git/db` and `/usr/local/cargo/registry`. Cargo
+#   downloads dependencies here. These downloads should not appear in the final
+#   image and can be cached between invocations of `docker build`.
+# - Cache `/cargo-mutants`. Cargo writes compilation artifacts here (see
+#   `--target-dir` argument). These artifacts should not appear in the final
+#   image and can be cached between invocations of `docker build`.
+# - Mask `/tmp`. We depend on `rustix`, which writes a file here when its build
+#   script runs. This file should not appear in the final image. See
+#   https://github.com/bytecodealliance/rustix/blob/035acdc416d9465abc937bd8cd6a0031afc170aa/build.rs#L248.
+RUN --mount=type=cache,target=/usr/local/cargo/git/db \
+    --mount=type=cache,target=/usr/local/cargo/registry \
+    --mount=type=cache,target=/cargo-mutants \
+    --mount=type=tmpfs,target=/tmp \
+    cargo install \
+        --locked \
+        --target-dir /cargo-mutants \
+        "cargo-mutants@$CARGO_MUTANTS_VERSION"
+
+WORKDIR /app
+
+ENTRYPOINT [ "cargo", "mutants" ]