Try this.

.github/workflows/docker-build-master.yaml

@@ -35,4 +35,4 @@ jobs:
           tags: registry.nova.do/shairport-sync:unstable-${{ env.BRANCH }}
           build-args: |
             BRANCH=${{ env.BRANCH }}
-          ssh: default=${{ secrets.SSH_PRIVATE_KEY }}
+          ssh: ${{ secrets.SSH_PRIVATE_KEY }}

docker/Dockerfile

@@ -1,5 +1,3 @@
-# syntax=docker/dockerfile:experimental
-
 FROM alpine:3.12 AS builder-base
 # Version 3.12 is required for now since libplist has breaking changes in v3.13 and above which are not yet standard
 # in debian/ubuntu.
@@ -51,8 +49,6 @@ COPY --from=builder-alac /usr/local/lib/libalac.* /usr/local/lib/
 COPY --from=builder-alac /usr/local/lib/pkgconfig/alac.pc /usr/local/lib/pkgconfig/alac.pc
 COPY --from=builder-alac /usr/local/include /usr/local/include
 
-RUN mkdir -p -m 0600 ~/.ssh && ssh-keyscan github.com >> ~/.ssh/known_hosts
-
 RUN --mount=type=ssh git clone git@github.com:charlesomer/nqptp.git
 WORKDIR /nqptp
 # Use development branch for now.