Restrict docker-api.0.2.2 to older OCaml

Fails on 5.1 (and most likely 5.0?) with

> File "../compatibility.ml", line 39, characters 4-15:
> 39 | output_file fname_out s
> ^^^^^^^^^^^
> Warning 6 [labels-omitted]: label content was omitted in the application of this function.
>
> File "../compatibility.ml", line 43, characters 4-15:
> 43 | output_file fname_out (input_file fname_in)
> ^^^^^^^^^^^
> Warning 6 [labels-omitted]: label content was omitted in the application of this function.
>
> File "../compatibility.ml", line 55, characters 2-13:
> 55 | output_file fname_out s
> ^^^^^^^^^^^
> Warning 6 [labels-omitted]: label content was omitted in the application of this function.
> (cd _build/default && /home/opam/.opam/5.1/bin/ocamlc.opt -w -40 -g -bin-annot -I src/.docker.objs/byte -I /home/opam/.opam/5.1/lib/bytes -I /home/opam/.opam/5.1/lib/ocaml/unix -I /home/opam/.opam/5.1/lib/seq -I /home/opam/.opam/5.1/lib/yojson -intf-suffix .ml -no-alias-deps -open Docker__ -o src/.docker.objs/byte/docker.cmo -c -impl src/docker.ml)
> File "src/docker.ml", line 46, characters 32-33:
> 46 | Buffer.add_subbytes buf b 0 r
> ^
> Error: This expression has type bytes but an expression was expected of type
> string
> (cd _build/default && /home/opam/.opam/5.1/bin/ocamlopt.opt -w -40 -g -I src/.docker.objs/byte -I src/.docker.objs/native -I /home/opam/.opam/5.1/lib/bytes -I /home/opam/.opam/5.1/lib/ocaml/unix -I /home/opam/.opam/5.1/lib/seq -I /home/opam/.opam/5.1/lib/yojson -intf-suffix .ml -no-alias-deps -open Docker__ -o src/.docker.objs/native/docker.cmx -c -impl src/docker.ml)
> File "src/docker.ml", line 46, characters 32-33:
> 46 | Buffer.add_subbytes buf b 0 r
> ^
> Error: This expression has type bytes but an expression was expected of type
> string