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Fix Heroku container push failing with "unsupported" media type #57

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dentarg merged 1 commit into from
Aug 18, 2026
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Fix Heroku container push failing with "unsupported" media type #57

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5 changes: 1 addition & 4 deletions .github/workflows/heroku-container.yml
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Original file line number Diff line number Diff line change
Expand Up @@ -81,8 +81,6 @@ jobs:
password: ${{ secrets.heroku-key }}

- uses: docker/setup-buildx-action@bb05f3f5519dd87d3ba754cc423b652a5edd6d2c # v4.2.0
with:
driver-opts: image=moby/buildkit:v0.30.0

- name: Build and push per target
env:
Expand Down Expand Up @@ -112,11 +110,10 @@ jobs:
for target in $TARGETS; do
docker buildx build \
--target "$target" \
--tag "registry.heroku.com/$APP/$target" \
--output "type=image,name=registry.heroku.com/$APP/$target,push=true,oci-mediatypes=false" \

@dentarg dentarg Aug 17, 2026

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Codex (gpt-5.6-sol) says

The review comment is incorrect. The workflow uses Buildx’s docker-container driver, whose default-load setting is false, so type=image,push=true does not load the image into the runner’s Docker image store. Docker also documents type=registry as identical to type=image, except that it implicitly enables push=true; it does not avoid separate storage behavior. (Docker exporter docs (https://docs.docker.com/build/exporters/image-registry/), driver docs (https://docs.docker.com/build/builders/drivers/docker-container/))

Using type=registry would be a reasonable syntactic simplification, but the claimed disk/time improvement is not valid.

--cache-from type=gha \
--cache-to type=gha,mode=max \
--provenance=false \
--push \
"${args[@]}" \
"${secrets[@]}" \
.
Expand Down