Do "--cache-from" and "--cache-to" really require "--layers"?

[flixman]

Issue Description

In the documentation for --cache-from and --cache-to is stated that --layers is required. However:

1. ... I have observed that --layers default value is true
2. ... I have run a couple of tests with and without --layers being set, and the results are the same: caches are reported to be used

without layers:

$ podman images --format "{{.Repository}}:{{.Tag}}" | grep 'registry.org/lists/builder' | xargs -r podman rmi
$ podman system prune -f
$ time podman build -f docker/lists/ci/Dockerfile -t registry.org/lists/lists:test --cache-from registry.org/lists/builder --cache-to registry.org/lists/builder .
STEP 1/4: FROM registry.org/lists/lists:acc
STEP 2/4: ADD requirements.dev.txt /tmp
--> Cache pulled from remote registry.org/lists/builder:95a4650b539ccea9d93d38c7d1ee6a7e80fa52e3d975097fe7e8af6b1db6053c:95a4650b539ccea9d93d38c7d1ee6a7e80fa52e3d975097fe7e8af6b1db6053c
--> Using cache 5a0e4396325023047038d3d49e7a68a03d35d33bb989136307b63bc148c4b965
--> 5a0e43963250
STEP 3/4: RUN --mount=type=cache,target=/root/.cache     apk add --no-cache --virtual devreqs gcc python3-dev musl-dev linux-headers &&     pip3 install -t /app/python_modules_test -r /tmp/requirements.dev.txt --no-compile &&     apk del devreqs
--> Cache pulled from remote registry.org/lists/builder:72229fe5ee2420647a548eb64a9cca511875b4c9f793fb990adbe7facaa1627f:72229fe5ee2420647a548eb64a9cca511875b4c9f793fb990adbe7facaa1627f
--> Using cache 265709f9d4506055aa03577e6bc0791ba2be165871259f39bb549ef3c1a5d6f2
--> 265709f9d450
STEP 4/4: ENV PYTHONPATH="${PYTHONPATH}:/app/python_modules_test" PATH="${PATH}:/app/python_modules_test/bin"
--> Using cache 76e6d4f2beace6f341d344243be60d64c273bfdaf66860b82eb3f6fa00ced83c
COMMIT registry.org/lists/lists:test
--> Pushing cache [registry.org/lists/builder]:894d3a64c54930b8cf90f268c16bf42684c57be17241fb1ad074487a8e867727
--> 76e6d4f2beac
Successfully tagged registry.org/lists/lists:test
76e6d4f2beace6f341d344243be60d64c273bfdaf66860b82eb3f6fa00ced83c

real    0m1.775s
user    0m0.376s
sys     0m0.280s

with --layers

$ podman images --format "{{.Repository}}:{{.Tag}}" | grep 'registry.org/lists/builder' | xargs -r podman rmi
$ podman system prune -f
$ time podman build -f docker/lists/ci/Dockerfile -t registry.org/lists/lists:test --cache-from registry.org/lists/builder --cache-to registry.org/lists/builder --layers .
STEP 1/4: FROM registry.org/lists/lists:acc
STEP 2/4: ADD requirements.dev.txt /tmp
--> Cache pulled from remote registry.org/lists/builder:95a4650b539ccea9d93d38c7d1ee6a7e80fa52e3d975097fe7e8af6b1db6053c:95a4650b539ccea9d93d38c7d1ee6a7e80fa52e3d975097fe7e8af6b1db6053c
--> Using cache 5a0e4396325023047038d3d49e7a68a03d35d33bb989136307b63bc148c4b965
--> 5a0e43963250
STEP 3/4: RUN --mount=type=cache,target=/root/.cache     apk add --no-cache --virtual devreqs gcc python3-dev musl-dev linux-headers &&     pip3 install -t /app/python_modules_test -r /tmp/requirements.dev.txt --no-compile &&     apk del devreqs
--> Cache pulled from remote registry.org/lists/builder:72229fe5ee2420647a548eb64a9cca511875b4c9f793fb990adbe7facaa1627f:72229fe5ee2420647a548eb64a9cca511875b4c9f793fb990adbe7facaa1627f
--> Using cache 265709f9d4506055aa03577e6bc0791ba2be165871259f39bb549ef3c1a5d6f2
--> 265709f9d450
STEP 4/4: ENV PYTHONPATH="${PYTHONPATH}:/app/python_modules_test" PATH="${PATH}:/app/python_modules_test/bin"
--> Using cache 76e6d4f2beace6f341d344243be60d64c273bfdaf66860b82eb3f6fa00ced83c
COMMIT registry.org/lists/lists:test
--> Pushing cache [registry.org/lists/builder]:894d3a64c54930b8cf90f268c16bf42684c57be17241fb1ad074487a8e867727
--> 76e6d4f2beac
Successfully tagged registry.org/lists/lists:test
76e6d4f2beace6f341d344243be60d64c273bfdaf66860b82eb3f6fa00ced83c

real    0m1.828s
user    0m0.395s
sys     0m0.231s

if cache is not used (so no --cache-from, --cache-to, or --layers flags are set), the build takes 1 minute.

Is this a documentation error?

Steps to reproduce the issue

There is no issue, is a documentation question.

Describe the results you received

There is no issue, is a documentation question.

Describe the results you expected

There is no issue, is a documentation question.

podman info output

host:
  arch: amd64
  buildahVersion: 1.38.0
  cgroupControllers:
  - cpu
  - memory
  - pids
  cgroupManager: systemd
  cgroupVersion: v2
  conmon:
    package: conmon-1:2.1.12-1
    path: /usr/bin/conmon
    version: 'conmon version 2.1.12, commit: e8896631295ccb0bfdda4284f1751be19b483264'
  cpuUtilization:
    idlePercent: 96.01
    systemPercent: 1.71
    userPercent: 2.27
  cpus: 16
  databaseBackend: sqlite
  distribution:
    distribution: arch
    version: unknown
  eventLogger: journald
  freeLocks: 1951
  hostname: altair
  idMappings:
    gidmap:
    - container_id: 0
      host_id: 1000
      size: 1
    - container_id: 1
      host_id: 100000
      size: 65536
    uidmap:
    - container_id: 0
      host_id: 1000
      size: 1
    - container_id: 1
      host_id: 100000
      size: 65536
  kernel: 6.12.6-arch1-1
  linkmode: dynamic
  logDriver: journald
  memFree: 6756691968
  memTotal: 15907188736
  networkBackend: netavark
  networkBackendInfo:
    backend: netavark
    dns:
      package: aardvark-dns-1.13.1-1
      path: /usr/lib/podman/aardvark-dns
      version: aardvark-dns 1.13.1
    package: netavark-1.13.1-1
    path: /usr/lib/podman/netavark
    version: netavark 1.13.1
  ociRuntime:
    name: crun
    package: crun-1.19.1-2
    path: /usr/bin/crun
    version: |-
      crun version 1.19.1
      commit: 3e32a70c93f5aa5fea69b50256cca7fd4aa23c80
      rundir: /run/user/1000/crun
      spec: 1.0.0
      +SYSTEMD +SELINUX +APPARMOR +CAP +SECCOMP +EBPF +CRIU +YAJL
  os: linux
  pasta:
    executable: /usr/bin/pasta
    package: passt-2024_11_27.c0fbc7e-1
    version: |
      pasta 2024_11_27.c0fbc7e
      Copyright Red Hat
      GNU General Public License, version 2 or later
        <https://www.gnu.org/licenses/old-licenses/gpl-2.0.html>
      This is free software: you are free to change and redistribute it.
      There is NO WARRANTY, to the extent permitted by law.
  remoteSocket:
    exists: true
    path: /run/user/1000/podman/podman.sock
  rootlessNetworkCmd: pasta
  security:
    apparmorEnabled: false
    capabilities: CAP_CHOWN,CAP_DAC_OVERRIDE,CAP_FOWNER,CAP_FSETID,CAP_KILL,CAP_NET_BIND_SERVICE,CAP_SETFCAP,CAP_SETGID,CAP_SETPCAP,CAP_SETUID,CAP_SYS_CHROOT
    rootless: true
    seccompEnabled: true
    seccompProfilePath: /etc/containers/seccomp.json
    selinuxEnabled: false
  serviceIsRemote: false
  slirp4netns:
    executable: ""
    package: ""
    version: ""
  swapFree: 0
  swapTotal: 0
  uptime: 182h 4m 58.00s (Approximately 7.58 days)
  variant: ""
plugins:
  authorization: null
  log:
  - k8s-file
  - none
  - passthrough
  - journald
  network:
  - bridge
  - macvlan
  - ipvlan
  volume:
  - local
registries:
  docker.io:
    Blocked: false
    Insecure: false
    Location: docker.io
    MirrorByDigestOnly: false
    Mirrors:
    - Insecure: false
      Location: registry.org
      PullFromMirror: ""
    Prefix: docker.io
    PullFromMirror: ""
  search:
  - docker.io
store:
  configFile: /home/user/.config/containers/storage.conf
  containerStore:
    number: 0
    paused: 0
    running: 0
    stopped: 0
  graphDriverName: overlay
  graphOptions: {}
  graphRoot: /home/user/.local/share/containers/storage
  graphRootAllocated: 500856545280
  graphRootUsed: 287223005184
  graphStatus:
    Backing Filesystem: extfs
    Native Overlay Diff: "false"
    Supports d_type: "true"
    Supports shifting: "true"
    Supports volatile: "true"
    Using metacopy: "false"
  imageCopyTmpDir: /var/tmp
  imageStore:
    number: 91
  runRoot: /run/user/1000/containers
  transientStore: false
  volumePath: /home/user/.local/share/containers/storage/volumes
version:
  APIVersion: 5.3.1
  Built: 1732225906
  BuiltTime: Thu Nov 21 22:51:46 2024
  GitCommit: 4cbdfde5d862dcdbe450c0f1d76ad75360f67a3c
  GoVersion: go1.23.3
  Os: linux
  OsArch: linux/amd64
  Version: 5.3.1

Podman in a container

No

Privileged Or Rootless

Rootless

Upstream Latest Release

Yes

Additional environment details

Additional environment details

Additional information

Additional information like issue happens only occasionally or issue happens with a particular architecture or on a particular setting