feat(library): Add support for Caddy

Introduce Caddy bincompat run. Build Caddy as static PIE using a
Dockerfile, inspired by Caddy's own Docker setup. Use `xcaddy` to build.
The build uses binary compatibility mode (i.e. the `base` image).

Add:

* `Kraftfile`: build / run rules, including pulling the `base` image
* `Dockerfile`: base filesystem, with binary and libraries
* `README.md`: document how to use
* `Caddyfile`: Caddy configuration file

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

library/caddy/2.7/Caddyfile

@@ -0,0 +1,3 @@
+:2015
+
+respond "Hello, world!"

library/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 /caddy
+COPY Caddyfile /Caddyfile

library/caddy/2.7/Kraftfile

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+spec: v0.6
+
+runtime: unikraft.org/base:latest
+
+rootfs: ./Dockerfile
+
+cmd: ["/caddy", "run"]

library/caddy/2.7/README.md

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+# Caddy 2.7
+
+This directory contains the definition for the `unikraft.org/caddy:2.7` image.
+
+To run this image, [install Unikraft's companion command-line toolchain `kraft`](https://unikraft.org/docs/cli) and then you can run:
+
+```
+kraft run unikraft.org/caddy:2.7 -p 2015:2015
+```
+
+Once executed, it will open port `2015` and wait for connections, and can be queried.
+
+## See also
+
+- [How to run unikernels locally in Unikraft's Documentation](https://unikraft.org/docs/cli/running).