Install pkg-config in builder images

[ShahanaFarooqui]

All three reproducible builds started failing after our last commit to pin coincurve builds from source. This is an environment gap in the repro images, not a problem with the coincurve pin itself.

coincurve==20.0.0 shipped prebuilt wheels, so nothing was ever compiled during uv sync. The git pin has no wheels, so the repro build now compiles coincurve from source, and its CMake build hard-requires pkg-config. Regular CI already installs pkg-config via .github/scripts/setup.sh, which is why PR CI passed; the repro Dockerfiles keep a separate apt list and were missing it.

Failed Action: https://github.com/ElementsProject/lightning/actions/runs/28993800082

Changelog-None.

[daywalker90]

The installation instructions also need updating. I took the liberty to update arch linux instructions as a whole, since they were horribly outdated. pkg-config command is provided by different package names, depending on the distro.

[madelinevibes]

LGTM! ebb8532 & 985d668

[daywalker90]

There were two problems with the docker build step. One was that it required the target arch's python headers and the other was that the vendored cmake is dynamically linked against libatomic1 which is usually included when installing build-essentials but we don't do that in that bookworm-slim image. So i kept it slim and only pulled in libatomic1.

[daywalker90]

 => [linux/amd64 lightningd 3/4] COPY --from=builder       /tmp/lightning_install/               /usr/local/                                                                                                                                        0.4s
 => [linux/amd64 lightningd 4/4] COPY tools/docker-entrypoint.sh    /entrypoint.sh                                                                                                                                                                  0.1s
 => ERROR [linux/amd64 builder 13/13] RUN find /tmp/lightning_install -type f -executable -exec     file {} + |     awk -F: '/ELF/ {print $1}' |     xargs -r arm-linux-gnueabihf-strip --strip-unneeded                                            0.7s
------
 > [linux/amd64 builder 13/13] RUN find /tmp/lightning_install -type f -executable -exec     file {} + |     awk -F: '/ELF/ {print $1}' |     xargs -r arm-linux-gnueabihf-strip --strip-unneeded:
0.383 arm-linux-gnueabihf-strip: /tmp/lightning_install/libexec/c-lightning/plugins/cln-lsps-client: file format not recognized
0.410 arm-linux-gnueabihf-strip: /tmp/lightning_install/libexec/c-lightning/plugins/clnrest: file format not recognized
0.454 arm-linux-gnueabihf-strip: /tmp/lightning_install/libexec/c-lightning/plugins/cln-bip353: file format not recognized
0.467 arm-linux-gnueabihf-strip: /tmp/lightning_install/libexec/c-lightning/plugins/cln-lsps-service: file format not recognized
0.474 arm-linux-gnueabihf-strip: /tmp/lightning_install/libexec/c-lightning/plugins/cln-currencyrate: file format not recognized
0.567 arm-linux-gnueabihf-strip: /tmp/lightning_install/libexec/c-lightning/plugins/wss-proxy: file format not recognized
------
WARNING: No output specified with docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load
WARNING: local cache import at /tmp/docker-cache skipped due to err: could not read /tmp/docker-cache/index.json: open /tmp/docker-cache/index.json: no such file or directory
ERROR: failed to build: failed to solve: process "/bin/bash -euo pipefail -c find /tmp/lightning_install -type f -executable -exec     file {} + |     awk -F: '/ELF/ {print $1}' |     xargs -r ${STRIP} --strip-unneeded" did not complete successfully: exit code: 123

The dependency issue is fixed but my run ends like this, not sure if that is ok.

What do we think about this approach?
#9303