feat(examples): Introduce Caddy as example

Introduce Caddy as binary compatibility run. Build Caddy as static PIE
application using `Dockerfile`. Then run it with the `base` kernel
images from `../../kernels/`.

Add typical files for a bincompat app:

* `Kraftfile`: build / run rules, including pulling the `base` image
* `Dockerfile`: filesystem, including binary and libraries
* `Makefile`: used to generate the root filesystem from the
  `Dockerfile`
  rules
* `README.md`: instructions to set up, build and run the application
* `config.yaml`: configuration file to generate scripts to the
  application
* `hosts`, `Caddyfile`: required Caddy configuration files

`config.yaml` is used to generate run scripts using the
`../../utils/bincompat/generate.py` script.

The kernels in `../../kernels` are generated by running the
`../../utils/bincompat/base-build-all.sh` script while inside the
`../../library/base/` directory.

Signed-off-by: Razvan Deaconescu <[email protected]>

examples/caddy-2.7/.gitignore

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+/rootfs/
+/rootfs.cpio
+/run-qemu*
+/run-fc*
+/kraft-run-*
+/fc*.json

examples/caddy-2.7/Caddyfile

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+:2015
+
+respond "Hello, World!"

examples/caddy-2.7/Dockerfile

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+FROM --platform=linux/x86_64 golang:1.21.4-bookworm AS build
+
+# Inspired from: https://github.com/caddyserver/caddy-docker/blob/master/2.7/builder/Dockerfile
+
+RUN set -xe; apt-get update ; \
+    apt-get install -y --no-install-recommends \
+    ca-certificates \
+    git \
+    libcap2-bin \
+    ;
+
+ENV XCADDY_VERSION v0.3.5
+# Configures xcaddy to build with this version of Caddy
+ENV CADDY_VERSION v2.7.5
+# Configures xcaddy to not clean up post-build (unnecessary in a container)
+ENV XCADDY_SKIP_CLEANUP 1
+# Sets capabilities for output caddy binary to be able to bind to privileged ports
+ENV XCADDY_SETCAP 1
+
+RUN set -xe; \
+    wget -O /tmp/xcaddy.tar.gz "https://github.com/caddyserver/xcaddy/releases/download/v0.3.5/xcaddy_0.3.5_linux_amd64.tar.gz"; \
+    tar x -z -f /tmp/xcaddy.tar.gz -C /usr/bin xcaddy; \
+    rm -f /tmp/xcaddy.tar.gz; \
+    chmod +x /usr/bin/xcaddy;
+
+ENV CGO_ENABLED=1
+ENV XCADDY_GO_BUILD_FLAGS "-buildmode=pie -ldflags '-linkmode external -extldflags -static-pie'"
+
+RUN /usr/bin/xcaddy build
+
+FROM scratch
+
+COPY --from=build /go/caddy /usr/local/bin/caddy
+COPY Caddyfile /etc/caddy/Caddyfile

examples/caddy-2.7/Kraftfile

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+spec: v0.6
+
+name: caddy
+
+runtime: index.unikraft.io/unikraft.org/base:latest
+
+rootfs: ./rootfs
+
+cmd: ["/usr/local/bin/caddy", "run", "--config", "/etc/caddy/Caddyfile"]

examples/caddy-2.7/Makefile

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+IMAGE_NAME = unikraft-caddy
+
+include ../../utils/bincompat/docker.Makefile
+
+export: clean create cleanfs
+	mkdir rootfs
+	docker export $(CONTAINER_NAME) | tar -x -C rootfs
+	cp hosts rootfs/etc/hosts

examples/caddy-2.7/README.md

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+# Caddy 2.7
+
+This directory contains the definition to run [Caddy](https://github.com/caddyserver/caddy) on Unikraft in binary compatibility mode.
+
+Follow the instructions in the common `../README.md` file to set up and configure the application.
+
+## Quick Run
+
+Use `kraft` to run the image:
+
+```console
+kraft run -M 256M -p 2015:2015
+```
+
+Once executed, it will open port `2015` and wait for connections, and can be queried.
+
+Query the service using:
+
+```console
+curl localhost:2015
+```
+
+This doesn't work, however, probably due to some Caddy specifics.
+
+## Scripted Run
+
+Use the scripted runs, detailed in the common `../README.md`.
+Once you run the the scripts, query the service:
+
+```console
+curl 172.44.0.2:2015
+```
+
+This will show a `Hello, World!` message, provided by the Caddy, based on the configuration on `Caddyfile`.

examples/caddy-2.7/config.yaml

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+networking: True
+accel: True
+memory: 512

examples/caddy-2.7/hosts

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+127.0.0.1	localhost
+::1     ip6-localhost ip6-loopback