Build 32bit on CI

.dockerignore

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+_build
+.git
+async
+mirage
+h2-async.opam
+h2-mirage.opam

.github/workflows/test_32bit.yml

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+name: 'Build & Test (32 bit)'
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+
+jobs:
+  docker-32bit:
+    runs-on: ubuntu-latest
+    container:
+      image: ocurrent/opam@sha256:e7a75342370514e1adf20c6c95102bb37dddefd5ad205c457e4aa72012283960
+    steps:
+    - uses: actions/checkout@v1
+    - name: Print the result of `uname -a`
+      run: uname -a
+    - name: apt-get update
+      run: sudo apt-get update && sudo apt-get upgrade -y
+    - name: opam update
+      run: opam update && opam upgrade
+    - name: opam depext conf-pkg-config conf-gmp conf-m4
+      run: opam pin add . -n
+    - name: Install OPAM dependencies
+      run: opam install -y --deps-only . -t
+    - name:
+      run: opam install -y --deps-only . -t
+    - name:
+      run: opam exec -- dune runtest

Dockerfile.32bit

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+# https://discuss.ocaml.org/t/how-to-generate-a-32-bit-binary-on-a-64-bit-system/6163/6
+FROM ocurrent/opam@sha256:e7a75342370514e1adf20c6c95102bb37dddefd5ad205c457e4aa72012283960
+
+WORKDIR /home/opam/src
+
+RUN sudo apt-get update && sudo apt-get upgrade -y
+RUN sudo apt-get install vim -y
+
+RUN opam update && opam upgrade
+
+RUN opam depext conf-pkg-config conf-gmp conf-m4
+RUN opam install h2 h2-lwt-unix --deps-only -t
+
+COPY --chown=opam . /home/opam/src
+
+# ENTRYPOINT
+