adds building oci containers

* Uses the haskell.nix container building infrastructure

* Is convoluted, containers are not directly built, only the manifest,
  or here a copy-to system

* Resulting tar is about 32 MByte, not tested yet

.github/workflows/ci-oci.yaml

@@ -0,0 +1,48 @@
+# https://github.com/marketplace/actions/install-nix
+
+# Builds oci container in a rather convoluted way.
+
+name: "ci-oci"
+on:
+  pull_request:
+  push:
+
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+
+    - uses: actions/checkout@v4
+
+    - name: Install nix
+      uses: cachix/install-nix-action@v27
+
+    # https://github.com/marketplace/actions/cache-nix-store
+    - name: Restore and cache Nix store
+      uses: nix-community/cache-nix-action@v5
+      with:
+        # restore and save a cache using this key
+        # TODO: Also hash the cabal.config and *cabal files, since we depend on them
+        primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix') }}
+        # if there's no cache hit, restore a cache by this prefix
+        restore-prefixes-first-match: nix-${{ runner.os }}-
+        # collect garbage until Nix store size (in bytes) is at most this number
+        # before trying to save a new cache
+        gc-max-store-size-linux: 1073741824
+        # do purge caches
+        purge: true
+        # purge all versions of the cache
+        purge-prefixes: cache-${{ runner.os }}-
+        # created more than this number of seconds ago relative to the start of the `Post Restore` phase
+        purge-created: 0
+        # except the version with the `primary-key`, if it exists
+        purge-primary-key: never
+
+    - run: nix build --accept-flake-config .#containers.x86_64-linux.wst.copyTo
+    - run: ./result/bin/copy-to oci-archive:oci.tar
+    - run: ls -alh oci.tar
+

nix/containers.nix

@@ -0,0 +1,18 @@
+{ repoRoot, inputs, pkgs, lib, system }:
+{
+
+  # Builds a docker container for the cabal executable given as input. First we
+  # build the container json itself. Note the explicit architecture.
+  #
+  # $ nix build .#containers.x86_64-linux.wst
+  #
+  wst = lib.iogx.mkContainerFromCabalExe {
+    exe = inputs.self.packages.wst-poc;
+    name = "wst-poc";
+    description = "WST Proof of Concept";
+    packages = [ ];
+    sourceUrl = "https://github.com/input-output-hk/wsc-poc";
+  };
+
+}
+

nix/outputs.nix

@@ -1,9 +1,9 @@
 { repoRoot, inputs, pkgs, lib, system }:
 let
   project = repoRoot.nix.project;
+  containers = repoRoot.nix.containers;
 in
 [
-  (
-    project.flake
-  )
+  ( project.flake )
+  { inherit containers; }
 ]