build(docker): Use opam-2.3 and run opam init --reinit -ni first

Otherwise, we get:
```
The following dependencies couldn't be met:
  - learn-ocaml -> mirage-runtime = 4.8.2
      unmet availability conditions: opam-version >= "2.1.0"
```
Related: https://github.com/ocaml-sf/learn-ocaml/actions/runs/15421443343/job/43397321956?pr=610#step:3:163
Related: ocurrent/docker-base-images#132
Related: https://github.com/ocurrent/docker-base-images/blob/02a40d239bbb8dd2700d9bfaa7511926a84eca7b/Dockerfile#L3

Author: erikmd

Dockerfile

@@ -5,6 +5,7 @@ WORKDIR /home/opam/learn-ocaml
 
 COPY learn-ocaml.opam learn-ocaml.opam.locked learn-ocaml-client.opam learn-ocaml-client.opam.locked ./
 RUN sudo chown -R opam:nogroup .
+RUN sudo ln -sf /usr/bin/opam-2.3 /usr/bin/opam && opam init --reinit -ni
 
 ENV OPAMYES true
 RUN echo 'archive-mirrors: [ "https://opam.ocaml.org/cache" ]' >> ~/.opam/config \

Dockerfile.test-client

@@ -9,6 +9,7 @@ WORKDIR /home/opam/learn-ocaml
 # Note: don't copy learn-ocaml.opam.locked
 COPY learn-ocaml-client.opam learn-ocaml.opam ./
 RUN sudo chown -R opam:nogroup .
+RUN sudo ln -sf /usr/bin/opam-2.3 /usr/bin/opam && opam init --reinit -ni
 
 ENV OPAMYES true
 RUN echo 'archive-mirrors: [ "https://opam.ocaml.org/cache" ]' >> ~/.opam/config \

Dockerfile.test-server

@@ -9,6 +9,7 @@ WORKDIR /home/opam/learn-ocaml
 # Note: don't copy learn-ocaml.locked
 COPY learn-ocaml.opam learn-ocaml-client.opam ./
 RUN sudo chown -R opam:nogroup .
+RUN sudo ln -sf /usr/bin/opam-2.3 /usr/bin/opam && opam init --reinit -ni
 
 ENV OPAMYES true
 RUN echo 'archive-mirrors: [ "https://opam.ocaml.org/cache" ]' >> ~/.opam/config \