Use BuildKit syntax in Dockerfile

Should speed up builds a bit.

Dockerfile

@@ -1,49 +1,59 @@
-FROM ocaml/opam:alpine-3.20-ocaml-5.2 as build
+# syntax=docker/dockerfile:1
+FROM ocaml/opam:alpine-3.20-ocaml-5.2 AS build
 
 # Install system dependencies
-RUN sudo apk update && sudo apk add --update libev-dev openssl-dev gmp-dev oniguruma-dev inotify-tools curl-dev autoconf
+RUN sudo apk add --no-cache \
+    autoconf \
+    curl-dev \
+    gmp-dev \
+    inotify-tools \
+    libev-dev \
+    oniguruma-dev \
+    openssl-dev
 
 # Branch freeze was opam-repo HEAD at the time of commit
-RUN cd opam-repository && git pull origin c45f5bab71d3589f41f9603daca5acad14df0ab0 && opam update
+RUN cd ~/opam-repository && git fetch -q origin master && git reset --hard 11bdbee61114a1cfa080b764e71c72a5760a93f0 && opam update
 
 WORKDIR /home/opam
 
-# Install Opam dependencies
-ADD ocamlorg.opam ocamlorg.opam
-RUN opam install . --deps-only
+# Install opam dependencies
+COPY --chown=opam --link ocamlorg.opam .
+RUN --mount=type=cache,target=/home/opam/.opam/download-cache,sharing=locked,uid=1000,gid=1000 \
+    opam install . --deps-only
 
 # Build project
-COPY --chown=opam:opam . .
+COPY --chown=opam . .
 RUN opam exec -- dune build @install --profile=release
 
 # Launch project in order to generate the package state cache
-RUN cd opam-repository && git checkout master && git pull origin master && opam update
-ENV OCAMLORG_REPO_PATH opam-repository
-ENV OCAMLORG_PKG_STATE_PATH package.state
+RUN cd ~/opam-repository && git checkout master && opam update
+ENV OCAMLORG_REPO_PATH=opam-repository
+ENV OCAMLORG_PKG_STATE_PATH=package.state
 RUN touch package.state && ./init-cache package.state
 
-FROM alpine:3.20 as run
+FROM alpine:3.20 AS run
 
-RUN apk update && apk add --update libev gmp git
+RUN apk add --no-cache \
+    git \
+    gmp \
+    libev
 
-RUN chmod -R 755 /var
+COPY --from=build --link /home/opam/package.state /var/package.state
+COPY --from=build --link /home/opam/opam-repository /var/opam-repository
+COPY --from=build --link /home/opam/_build/default/src/ocamlorg_web/bin/main.exe /bin/server
 
-COPY --from=build /home/opam/package.state /var/package.state
-COPY --from=build /home/opam/opam-repository /var/opam-repository
-COPY --from=build /home/opam/_build/default/src/ocamlorg_web/bin/main.exe /bin/server
+COPY --link playground/asset playground/asset
 
-COPY playground/asset playground/asset
-
-RUN git clone https://github.com/ocaml-web/html-compiler-manuals /manual
+ADD --keep-git-dir --link https://github.com/ocaml-web/html-compiler-manuals /manual
 
 RUN git config --global --add safe.directory /var/opam-repository
 
-ENV OCAMLORG_REPO_PATH /var/opam-repository/
-ENV OCAMLORG_MANUAL_PATH /manual
-ENV OCAMLORG_PKG_STATE_PATH /var/package.state
-ENV DREAM_VERBOSITY info
-ENV OCAMLORG_HTTP_PORT 8080
+ENV OCAMLORG_REPO_PATH=/var/opam-repository/
+ENV OCAMLORG_MANUAL_PATH=/manual
+ENV OCAMLORG_PKG_STATE_PATH=/var/package.state
+ENV DREAM_VERBOSITY=info
+ENV OCAMLORG_HTTP_PORT=8080
 
 EXPOSE 8080
 
-ENTRYPOINT /bin/server
+ENTRYPOINT ["/bin/server"]