refactor(docker): consume the base images instead of building them

The `virgin` stage installed the distribution build dependencies, and that
layer now lives in its own repository:
https://github.com/kevinveenbirkenbach/base-images. What remains here is one
image per distribution that installs pkgmgr on top of it. `slim` goes with it:
it was published for every distribution and pulled by nothing.

scripts/build/base.sh stops pinning five upstream images and resolves
ghcr.io/<owner>/base-<distro>:<tag> instead. Its env names follow the images
repository rather than this one, so the code shows whose namespace that is.
image.sh loses the --target axis, the -virgin/-slim tag suffixes with it, and
passes --platform on push. Manjaro joins the set; os_resolver.sh already maps
it onto arch, so it needs no dependency script of its own.

The two virgin workflows pull the base image rather than building it. What
they prove is unchanged: pkgmgr installs into an untouched container, as root
and as an unprivileged user.

test_distro_dependency_scripts_install_gpg_tools goes: it grepped the
dependency scripts for gnupg, and those scripts are base-images' responsibility
now, where the contract test runs `gpg --version` instead of reading a package
list.

Verified against locally built base images: the arch image builds FROM
base-arch, and the full suite passes inside it - 405 unit, 90 integration and
49 e2e tests.

Requires base-images to be published first; until then every build here fails
at the pull.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/build/image.sh

@@ -10,7 +10,6 @@ source "${SCRIPT_DIR}/base.sh"
 
 NO_CACHE=0
 MISSING_ONLY=0
-TARGET=""
 IMAGE_TAG=""         # local image name or base tag (without registry)
 PUSH=0               # if 1 -> use buildx and push (requires docker buildx)
 PUBLISH=0            # if 1 -> push with semantic tags (latest/version/stable + arch aliases)
@@ -23,9 +22,6 @@ DEFAULT_DISTRO="arch"
 
 usage() {
   local default_tag="pkgmgr-${PKGMGR_DISTRO}"
-  if [[ -n "${TARGET:-}" ]]; then
-    default_tag="${default_tag}-${TARGET}"
-  fi
 
   cat <<EOF
 Usage: PKGMGR_DISTRO=<distro> $0 [options]
@@ -33,7 +29,6 @@ Usage: PKGMGR_DISTRO=<distro> $0 [options]
 Build options:
   --missing             Build only if the image does not already exist (local build only)
   --no-cache            Build with --no-cache
-  --target <name>       Build a specific Dockerfile target (e.g. virgin, slim)
   --tag <image>         Override the output image tag (default: ${default_tag})
 
 Publish options:
@@ -47,7 +42,7 @@ Publish options:
 
 Notes:
 - --publish implies --push and requires --registry, --owner, and --version.
-- Local build (no --push) uses "docker build" and creates local images like "pkgmgr-arch" / "pkgmgr-arch-virgin" / "pkgmgr-arch-slim".
+- Local build (no --push) uses "docker build" and creates a local image like "pkgmgr-arch".
 EOF
 }
 
@@ -55,11 +50,6 @@ while [[ $# -gt 0 ]]; do
   case "$1" in
     --no-cache) NO_CACHE=1; shift ;;
     --missing)  MISSING_ONLY=1; shift ;;
-    --target)
-      TARGET="${2:-}"
-      [[ -n "${TARGET}" ]] || { echo "ERROR: --target requires a value (e.g. virgin|slim)"; exit 2; }
-      shift 2
-      ;;
     --tag)
       IMAGE_TAG="${2:-}"
       [[ -n "${IMAGE_TAG}" ]] || { echo "ERROR: --tag requires a value"; exit 2; }
@@ -104,9 +94,6 @@ done
 # Derive default local tag if not provided
 if [[ -z "${IMAGE_TAG}" ]]; then
   IMAGE_TAG="${REPO_PREFIX}-${PKGMGR_DISTRO}"
-  if [[ -n "${TARGET}" ]]; then
-    IMAGE_TAG="${IMAGE_TAG}-${TARGET}"
-  fi
 fi
 
 BASE_IMAGE="$(resolve_base_image "$PKGMGR_DISTRO")"
@@ -143,7 +130,7 @@ echo "------------------------------------------------------------"
 echo "[build] Building image"
 echo "distro     = ${PKGMGR_DISTRO}"
 echo "BASE_IMAGE = ${BASE_IMAGE}"
-if [[ -n "${TARGET}" ]]; then echo "target    = ${TARGET}"; fi
+echo "platforms = $(resolve_platforms "$PKGMGR_DISTRO")"
 if [[ "${NO_CACHE}" == "1" ]]; then echo "cache     = disabled"; fi
 if [[ "${PUSH}" == "1" ]]; then echo "push      = enabled"; fi
 if [[ "${PUBLISH}" == "1" ]]; then
@@ -162,23 +149,12 @@ if [[ "${NO_CACHE}" == "1" ]]; then
   build_args+=(--no-cache)
 fi
 
-if [[ -n "${TARGET}" ]]; then
-  build_args+=(--target "${TARGET}")
-fi
-
 compute_publish_tags() {
   local distro_tag_base="${REGISTRY}/${OWNER}/${REPO_PREFIX}-${PKGMGR_DISTRO}"
   local alias_tag_base=""
 
-  if [[ -n "${TARGET}" ]]; then
-    distro_tag_base="${distro_tag_base}-${TARGET}"
-  fi
-
   if [[ "${PKGMGR_DISTRO}" == "${DEFAULT_DISTRO}" ]]; then
     alias_tag_base="${REGISTRY}/${OWNER}/${REPO_PREFIX}"
-    if [[ -n "${TARGET}" ]]; then
-      alias_tag_base="${alias_tag_base}-${TARGET}"
-    fi
   fi
 
   local tags=()
@@ -211,6 +187,7 @@ if [[ "${PUSH}" == "1" ]]; then
     bx_args+=(-t "${IMAGE_TAG}")
   fi
 
+  bx_args+=(--platform "$(resolve_platforms "$PKGMGR_DISTRO")")
   bx_args+=("${build_args[@]}")
   bx_args+=(.)