Docker Build "apk add" Network Failure in CI

An apk add inside an Alpine build could not reach the package mirrors. The fetch fails with a temporary/network error - DNS, a flaky mirror, or no outbound network for that build step.

What this error means

A RUN apk add ... step fails with temporary error (try again later), Could not connect to ... dl-cdn.alpinelinux.org, or fetch ... failed. The same Dockerfile builds fine when the network is healthy.

docker

#6 1.20 fetch https://dl-cdn.alpinelinux.org/alpine/v3.20/main/x86_64/APKINDEX.tar.gz
#6 1.45 ERROR: unable to select packages:
#6 1.45   temporary error (try again later)

Common causes

A flaky or slow Alpine mirror

The CDN mirror can briefly fail or time out, so the index fetch (and thus package selection) fails. This is usually transient.

DNS resolution failing in the build

If the builder cannot resolve dl-cdn.alpinelinux.org, every fetch fails before it even connects.

No outbound network for the step

A --network=none step, or a runner with restricted egress, leaves apk with nothing to fetch.

How to fix it

Update the index and retry within the step

Refresh the index and let the package manager retry the fetch.

Dockerfile

RUN apk update && apk add --no-cache ca-certificates curl

Confirm egress and DNS for the build

Make sure the build step has network and can resolve the mirror.

How to prevent it

Use apk add --no-cache and keep package lists minimal.
Ensure the build step has outbound network and working DNS.
Consider a pinned internal mirror for high-volume CI.