*: configure nix binary cache

Add a [nix binary cache using S3 as the
backend](https://nix.dev/manual/nix/2.25/store/types/s3-binary-cache-store) for
CI builds. This shaves about 3m30s off each CI job.

(cherry picked from commit 96ef0e1)

Author: chrisseto

ci/docker/entrypoint.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+
+# NB: Only our CI agents push to this cache but we don't have PKI (e.g.
+# signing) setup for them.
+## So we mark the cache as trusted to skip signature verifications.
+CACHE="s3://redpanda-nix-cache-k8s-m6id12xlarge?region=us-west-2&trusted=true"
+
+nix develop --option extra-substituters "$CACHE" --impure --command "$@"
+EXIT_CODE="$?"
+
+echo "--- Pushing to :nix: cache"
+nix copy --to "$CACHE" .#devshell
+
+exit "$EXIT_CODE"

ci/docker/nix.Dockerfile

@@ -18,4 +18,6 @@ RUN echo 'experimental-features = nix-command flakes' >> /etc/nix/nix.conf \
 	# easier to disable this check entirely:
 	&& git config --global --add safe.directory '*'
 
-ENTRYPOINT ["nix", "develop", "--impure", "--command"]
+COPY ci/docker/entrypoint.sh /
+
+ENTRYPOINT ["bash", "/entrypoint.sh"]

ci/scripts/run-in-nix-docker.sh

@@ -33,7 +33,7 @@ fi
 set -e
 
 # BUILDKITE environment variables are used by `buildkite-agent` cli to upload artifact
-printenv | grep '^BUILDKITE.*=' > env-file-for-buildkite
+printenv | { grep '^BUILDKITE.*=' || true; } > env-file-for-buildkite
 
 # Build the base image and grab the SHA.
 IMAGE_SHA=$(docker build --quiet -f ./ci/docker/nix.Dockerfile .)

flake.nix

@@ -34,6 +34,10 @@
         {
           formatter = pkgs.nixpkgs-fmt;
 
+          # Make it possible to reference the devshell context from standard
+          # nix commands. e.g. nix copy .#devshell.
+          packages.devshell = self'.devShells.default;
+
           devshells.default = {
             env = [
               { name = "GOPRIVATE"; value = "github.com/redpanda-data/flux-controller-shim"; }

taskfiles/ci.yml

@@ -21,6 +21,7 @@ tasks:
   test:unit:
     cmds:
       - defer: 'buildkite-agent artifact upload "{{.SRC_DIR}}/unit-tests-*.xml"'
+      - 'echo "--- Running unit tests"'
       - task: :test:unit
         vars:
           GO_TEST_RUNNER: gotestsum --raw-command --junitfile=unit-tests-%.xml -- go test
@@ -30,6 +31,7 @@ tasks:
     cmds:
       - defer: 'kind delete clusters --all'
       - defer: 'buildkite-agent artifact upload "{{.SRC_DIR}}/integration-tests-*.xml"'
+      - 'echo "--- Running integration tests"'
       - task: :test:integration
         vars:
           GO_TEST_RUNNER: gotestsum --raw-command --junitfile=integration-tests-%.xml -- go test
@@ -39,6 +41,7 @@ tasks:
     cmds:
       - defer: 'kind delete clusters --all'
       - defer: 'buildkite-agent artifact upload "{{.SRC_DIR}}/acceptance-tests-*.xml"'
+      - 'echo "--- Running acceptance tests"'
       - task: :test:acceptance
         vars:
           GO_TEST_RUNNER: gotestsum --raw-command --junitfile=acceptance-tests-%.xml -- go test