Remove support for ARM64 platforms in build workflow

We cannot currently test _any_ ARM64 platforms under qemu because qemu
cannot currently support iptables.  See multiarch/qemu-user-static#191
for more details.

This is because this role actually starts the Docker service before
pre-downloading some Docker images, and starting the Docker service
requires some interaction with iptables.

.github/workflows/build.yml

@@ -179,7 +179,15 @@ jobs:
       matrix:
         architecture:
           - amd64
-          - arm64
+          # We cannot currently test _any_ ARM64 platforms under qemu
+          # because qemu cannot currently support iptables:
+          # https://github.com/multiarch/qemu-user-static/issues/191
+          #
+          # This is because this role actually starts the Docker
+          # service before pre-downloading some Docker images, and
+          # starting the Docker service requires some interaction with
+          # iptables.
+          # - arm64
         platform:
           # RedHat support was removed from this Ansible role because
           # some of its tasks are Debian-specific.

README.md

@@ -5,6 +5,14 @@
 
 An Ansible role for installing [cisagov/guacamole-composition](https://github.com/cisagov/guacamole-composition).
 
+> [!NOTE]
+> We cannot currently test *any* ARM64 platforms under `qemu` because
+> [`qemu` cannot currently support
+> `iptables`](https://github.com/multiarch/qemu-user-static/issues/191).
+> This is because this role actually starts the Docker service before
+> pre-downloading some Docker images, and starting the Docker service
+> requires some interaction with `iptables`.
+
 ## Requirements ##
 
 None.