Add dockerfiles for i586, riscv, and thumb

These targets are tested in `compiler-builtins`, but not yet `libm`. Add
dockerfiles to prepare for this.

Author: tgross35

ci/docker/i586-unknown-linux-gnu/Dockerfile

@@ -0,0 +1,5 @@
+FROM ubuntu:24.04
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    gcc-multilib libc6-dev ca-certificates

ci/docker/riscv64gc-unknown-linux-gnu/Dockerfile

@@ -0,0 +1,15 @@
+FROM ubuntu:24.04
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    gcc libc6-dev qemu-user-static ca-certificates \
+    gcc-riscv64-linux-gnu libc6-dev-riscv64-cross \
+    qemu-system-riscv64
+
+ENV TOOLCHAIN_PREFIX=riscv64-linux-gnu-
+ENV CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_LINKER="$TOOLCHAIN_PREFIX"gcc \
+    CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_RUNNER=qemu-riscv64-static \
+    AR_riscv64gc_unknown_linux_gnu="$TOOLCHAIN_PREFIX"ar \
+    CC_riscv64gc_unknown_linux_gnu="$TOOLCHAIN_PREFIX"gcc \
+    QEMU_LD_PREFIX=/usr/riscv64-linux-gnu \
+    RUST_TEST_THREADS=1

ci/docker/thumbv6m-none-eabi/Dockerfile

@@ -0,0 +1,9 @@
+ARG IMAGE=ubuntu:24.04
+FROM $IMAGE
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    gcc libc6-dev ca-certificates \
+    gcc-arm-none-eabi \
+    libnewlib-arm-none-eabi
+ENV BUILD_ONLY=1

ci/docker/thumbv7em-none-eabi/Dockerfile

@@ -0,0 +1,9 @@
+ARG IMAGE=ubuntu:24.04
+FROM $IMAGE
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    gcc libc6-dev ca-certificates \
+    gcc-arm-none-eabi \
+    libnewlib-arm-none-eabi
+ENV BUILD_ONLY=1

ci/docker/thumbv7em-none-eabihf/Dockerfile

@@ -0,0 +1,9 @@
+ARG IMAGE=ubuntu:24.04
+FROM $IMAGE
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    gcc libc6-dev ca-certificates \
+    gcc-arm-none-eabi \
+    libnewlib-arm-none-eabi
+ENV BUILD_ONLY=1

ci/docker/thumbv7m-none-eabi/Dockerfile

@@ -0,0 +1,9 @@
+ARG IMAGE=ubuntu:24.04
+FROM $IMAGE
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    gcc libc6-dev ca-certificates \
+    gcc-arm-none-eabi \
+    libnewlib-arm-none-eabi
+ENV BUILD_ONLY=1