fix: pass explicit docker runtime in acceptance tests (#1160)

## Summary

- Adds optional second argument to `build-lgtm.sh` for explicit runtime
override (e.g. `./build-lgtm.sh latest docker`)
- Acceptance tests now pass `docker` explicitly, because `oats`
hardcodes `docker compose` to orchestrate services

## Problem

On CI runners (Ubuntu 24.04) where both Docker and Podman are installed,
if `build-lgtm.sh` uses Podman but `oats` runs `docker compose`, the
built image is invisible to Docker — causing acceptance test timeouts
(connection refused).

This is currently not an issue on `main` (Docker-first preference), but
becomes one when #1158 lands (Podman-first preference). This PR makes
the acceptance tests resilient regardless of the default preference
order.

## Test plan

- [ ] CI acceptance tests pass on this branch
- [ ] `./build-lgtm.sh latest` still auto-detects (no regression)
- [ ] `./build-lgtm.sh latest docker` forces Docker
- [ ] `./build-lgtm.sh latest podman` forces Podman

Author: zeitlinger

.mise/tasks/acceptance-tests.sh

@@ -8,5 +8,6 @@ version=${usage_version:-$(date +%Y%m%d%H%M%S)}
 
 echo "using version $version"
 
-mise run build-lgtm "$version"
+# Force Docker: oats hardcodes `docker compose`, so the image must be built with Docker.
+./build-lgtm.sh "$version" docker
 oats -timeout 5m -lgtm-version "$version" examples/

build-lgtm.sh

@@ -3,10 +3,19 @@
 set -euo pipefail
 
 RELEASE=${1:-latest}
+CONTAINER_RUNTIME_OVERRIDE=${2:-}
 
 echo "Building the Grafana OTEL-LGTM image with release ${RELEASE}..."
 
-if command -v docker >/dev/null 2>&1; then
+if [ -n "$CONTAINER_RUNTIME_OVERRIDE" ]; then
+	case "$CONTAINER_RUNTIME_OVERRIDE" in
+	docker | podman) RUNTIME="$CONTAINER_RUNTIME_OVERRIDE" ;;
+	*)
+		echo "Invalid runtime: $CONTAINER_RUNTIME_OVERRIDE (must be docker or podman)"
+		exit 1
+		;;
+	esac
+elif command -v docker >/dev/null 2>&1; then
 	RUNTIME=docker
 elif command -v podman >/dev/null 2>&1; then
 	RUNTIME=podman
@@ -15,4 +24,4 @@ else
 	exit 1
 fi
 
-$RUNTIME buildx build -f docker/Dockerfile docker --tag grafana/otel-lgtm:"${RELEASE}" --build-arg LGTM_VERSION="${RELEASE}"
+"$RUNTIME" buildx build -f docker/Dockerfile docker --tag grafana/otel-lgtm:"${RELEASE}" --build-arg LGTM_VERSION="${RELEASE}"