--cache-to causes compression_format from containers.conf to be ignored

[jiridanek]

Description

When using podman build --cache-to, the compression_format setting from containers.conf is ignored for both the cache layers AND the final image. Without --cache-to, the compression format is respected correctly.

Steps to Reproduce

1. Create a minimal containers.conf with zstd:chunked:

[engine]
compression_format="zstd:chunked"

2. Create a minimal Dockerfile:

FROM alpine:latest
RUN echo "layer1" > /layer1.txt
RUN echo "layer2" > /layer2.txt

3. Build and push WITHOUT --cache-to:

podman build --no-cache --layers -t quay.io/jdanek/cache-compression-test:v8 .
podman push quay.io/jdanek/cache-compression-test:v8
crane manifest quay.io/jdanek/cache-compression-test:v8 | jq -r '.layers[].mediaType'

Result: application/vnd.oci.image.layer.v1.tar+zstd ✓ (correct)

4. Build and push WITH --cache-to:

podman system reset -f
podman build --no-cache --layers --cache-to quay.io/jdanek/cache-compression-test-cache-v6 -t quay.io/jdanek/cache-compression-test:v7 .
podman push quay.io/jdanek/cache-compression-test:v7
crane manifest quay.io/jdanek/cache-compression-test:v7 | jq -r '.layers[].mediaType'

Result: application/vnd.oci.image.layer.v1.tar+gzip ✗ (should be zstd)

The cache also uses gzip:

crane manifest quay.io/jdanek/cache-compression-test-cache-v6:077cd4b1a9a353fc2e41beb42b498264aef7287aed9675da9899238a4ddec6cc | jq -r '.layers[].mediaType'

Result: application/vnd.oci.image.layer.v1.tar+gzip

Expected Behavior

Both cache and final image should use zstd:chunked compression as specified in containers.conf, regardless of whether --cache-to is used.

Actual Behavior

Scenario	Cache	Final Image
Build with --layers (no cache-to)	N/A	zstd ✓
Build with --layers --cache-to	gzip	gzip

The --cache-to flag causes the entire compression_format setting to be ignored.

Environment

podman version 5.7.1
macOS with podman machine (Fedora CoreOS VM)

containers.conf location

/etc/containers/containers.conf in the podman machine VM

Related Issues

• Buildah reuse gzip layers when "--compression-format zstd" assigned #4613 - Buildah reuse gzip layers when "--compression-format zstd" assigned (fixed)
• When layering on top of a compressed image, podman build remove the layers compression and annotations podman#26088 - podman build removes layer compression when layering on compressed images

Test Images

The test images are publicly available for verification:

• Cache: quay.io/jdanek/cache-compression-test-cache-v6
• Final (with cache-to, gzip): quay.io/jdanek/cache-compression-test:v7
• Final (without cache-to, zstd): quay.io/jdanek/cache-compression-test:v8

[jiridanek]

Additional Environment Information

podman version output

Client:        Podman Engine
Version:       5.7.1
API Version:   5.7.1
Go Version:    go1.25.5
Built:         Tue Dec  9 21:11:03 2025
Build Origin:  brew
OS/Arch:       darwin/arm64

Server:       Podman Engine
Version:      5.7.1
API Version:  5.7.1
Go Version:   go1.25.4 X:nodwarf5
Git Commit:   f845d14e941889ba4c071f35233d09b29d363c75
Built:        Tue Dec  9 01:00:00 2025
OS/Arch:      linux/arm64

podman info output

host:
  arch: arm64
  buildahVersion: 1.42.2
  cgroupManager: systemd
  cgroupVersion: v2
  conmon:
    package: conmon-2.1.13-2.fc43.aarch64
    path: /usr/bin/conmon
    version: 'conmon version 2.1.13, commit: '
  databaseBackend: sqlite
  distribution:
    distribution: fedora
    variant: coreos
    version: "43"
  kernel: 6.17.7-300.fc43.aarch64
  networkBackend: netavark
  ociRuntime:
    name: crun
    package: crun-1.24-1.fc43.aarch64
    path: /usr/bin/crun
    version: |-
      crun version 1.24
      commit: 54693209039e5e04cbe3c8b1cd5fe2301219f0a1
  os: linux
  rootlessNetworkCmd: pasta
  security:
    apparmorEnabled: false
    rootless: false
    seccompEnabled: true
    selinuxEnabled: true
store:
  configFile: /usr/share/containers/storage.conf
  graphDriverName: overlay
  graphOptions:
    overlay.additionalImageStores:
    - /usr/lib/containers/storage
    overlay.imagestore: /usr/lib/containers/storage
    overlay.mountopt: nodev,metacopy=on
  graphRoot: /var/lib/containers/storage
  graphStatus:
    Backing Filesystem: xfs
    Native Overlay Diff: "false"
    Supports d_type: "true"
    Supports shifting: "true"
    Supports volatile: "true"
    Using metacopy: "true"
version:
  APIVersion: 5.7.1
  BuildOrigin: 'Copr: packit/containers-podman-27732'
  Built: 1765238400
  GitCommit: f845d14e941889ba4c071f35233d09b29d363c75
  GoVersion: go1.25.4 X:nodwarf5
  Os: linux
  Version: 5.7.1

storage.conf

[storage]
driver = "overlay"
runroot = "/run/containers/storage"
graphroot = "/var/lib/containers/storage"

[storage.options]
additionalimagestores = [
  "/usr/lib/containers/storage"
]

[storage.options.overlay]
mountopt = "nodev,metacopy=on"

Upstream Latest Release

Yes - tested with podman 5.7.1 which includes buildah 1.42.2 (latest at time of testing).

Additional environment details

• macOS host (darwin/arm64) running podman machine
• Podman machine running Fedora CoreOS 43 (linux/arm64)
• Podman installed via Homebrew on macOS
• containers.conf with compression_format="zstd:chunked" placed in /etc/containers/containers.conf inside the podman machine VM