diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 379dce8..d5efec6 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -14,8 +14,6 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
-        ocaml-compiler:
-          - 4.14.x
 
     runs-on: ${{ matrix.os }}
 
@@ -23,19 +21,10 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
 
-      - name: Use OCaml ${{ matrix.ocaml-compiler }}
-        uses: ocaml/setup-ocaml@v2
+      - run: ./build-with-docker.sh
+
+      - name: Upload Artifact
+        uses: actions/upload-artifact@v3
         with:
-          ocaml-compiler: ${{ matrix.ocaml-compiler }}
-
-      - run: opam depext solo5 "mirage>4"
-
-      - run: opam install solo5 "mirage>4"
-
-      - run: opam exec -- mirage configure -t xen
-
-      - run: opam exec -- make depend
-
-      - run: opam exec -- dune build
-
-      - run: sha256sum dist/qubes-firewall.xen
+          name: mirage-firewall.tar.bz2
+          path: mirage-firewall.tar.bz2
diff --git a/Dockerfile b/Dockerfile
index 564f56e..c511cdb 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,20 +1,21 @@
 # Pin the base image to a specific hash for maximum reproducibility.
 # It will probably still work on newer images, though, unless an update
 # changes some compiler optimisations (unlikely).
-# fedora-35-ocaml-4.14
-FROM ocaml/opam@sha256:68b7ce1fd4c992d6f3bfc9b4b0a88ee572ced52427f0547b6e4eb6194415f585
-ENV PATH="${PATH}:/home/opam/.opam/4.14/bin"
+# ubuntu-20.04
+FROM ubuntu@sha256:b25ef49a40b7797937d0d23eca3b0a41701af6757afca23d504d50826f0b37ce
 
-# Since mirage 4.2 we must use opam version 2.1 or later
-RUN sudo ln -sf /usr/bin/opam-2.1 /usr/bin/opam
+RUN apt update && apt install --no-install-recommends --no-install-suggests -y wget ca-certificates git patch unzip make gcc g++ libc-dev
+RUN wget -O /usr/bin/opam https://github.com/ocaml/opam/releases/download/2.1.3/opam-2.1.3-i686-linux && chmod 755 /usr/bin/opam
 
+ENV OPAMROOT=/tmp
+ENV OPAMCONFIRMLEVEL=unsafe-yes
 # Pin last known-good version for reproducible builds.
 # Remove this line (and the base image pin above) if you want to test with the
 # latest versions.
-RUN cd /home/opam/opam-repository && git fetch origin master && git reset --hard 685eb4efcebfa671660e55d76dea017f00fed4d9 && opam update
-
-RUN opam install -y mirage opam-monorepo ocaml-solo5
-RUN mkdir /home/opam/qubes-mirage-firewall
-ADD config.ml /home/opam/qubes-mirage-firewall/config.ml
-WORKDIR /home/opam/qubes-mirage-firewall
-CMD opam exec -- mirage configure -t xen && make depend && make tar
+RUN opam init --disable-sandboxing -a --bare https://github.com/ocaml/opam-repository.git#685eb4efcebfa671660e55d76dea017f00fed4d9
+RUN opam switch create myswitch 4.14.0
+RUN opam exec -- opam install -y mirage opam-monorepo ocaml-solo5
+RUN mkdir /tmp/orb-build
+ADD config.ml /tmp/orb-build/config.ml
+WORKDIR /tmp/orb-build
+CMD opam exec -- sh -exc 'mirage configure -t xen --allocation-policy=best-fit && make depend && make tar'
diff --git a/Makefile.user b/Makefile.user
index 04d772b..fb04a23 100644
--- a/Makefile.user
+++ b/Makefile.user
@@ -1,6 +1,8 @@
 tar: build
 	rm -rf _build/mirage-firewall
 	mkdir _build/mirage-firewall
+	cp dist/qubes-firewall.xen dist/qubes-firewall.xen.debug
+	strip dist/qubes-firewall.xen
 	cp dist/qubes-firewall.xen _build/mirage-firewall/vmlinuz
 	touch _build/mirage-firewall/modules.img
 	cat /dev/null | gzip -n > _build/mirage-firewall/initramfs
diff --git a/build-with-docker.sh b/build-with-docker.sh
index e3ddce7..4dfbb34 100755
--- a/build-with-docker.sh
+++ b/build-with-docker.sh
@@ -3,7 +3,7 @@ set -eu
 echo Building Docker image with dependencies..
 docker build -t qubes-mirage-firewall .
 echo Building Firewall...
-docker run --rm -i -v `pwd`:/home/opam/qubes-mirage-firewall qubes-mirage-firewall
+docker run --rm -i -v `pwd`:/tmp/orb-build qubes-mirage-firewall
 echo "SHA2 of build:   $(sha256sum ./dist/qubes-firewall.xen)"
-echo "SHA2 last known: f499b2379c62917ac32854be63f201e6b90466e645e54dea51e376baccdf26ab"
+echo "SHA2 last known: 3f71a1b672a15d145c7d40405dd75f06a2b148d2cfa106dc136e3da38552de41"
 echo "(hashes should match for released versions)"