Dockerfile fails to build

[copy]

With the following error, which is ocaml-ppx/ocaml-migrate-parsetree#89:

#=== ERROR while compiling ocaml-migrate-parsetree.2.2.0 ======================#
# context              2.0.8 | linux/x86_64 | ocaml-variants.4.12.0+domains+effects | file:///home/opam/opam-repository
# path                 ~/.opam/4.12/.opam-switch/build/ocaml-migrate-parsetree.2.2.0
# command              ~/.opam/4.12/bin/dune build -p ocaml-migrate-parsetree -j 47
# exit-code            1
# env-file             ~/.opam/log/ocaml-migrate-parsetree-7-7681ba.env
# output-file          ~/.opam/log/ocaml-migrate-parsetree-7-7681ba.out
### output ###
#       ocamlc src/.migrate_parsetree.objs/byte/migrate_parsetree__Ast_412.{cmi,cmo,cmt} (exit 2)
# (cd _build/default && /home/opam/.opam/4.12/bin/ocamlc.opt -w -40 -g -bin-annot -I src/.migrate_parsetree.objs/byte -I /home/opam/.opam/4.12/lib/ocaml/compiler-libs -no-alias-deps -open Migrate_parsetree__ -o src/.migrate_parsetree.objs/byte/migrate_parsetree__Ast_412.cmo -c -impl src/ast_412.pp.ml)
# File "src/ast_412.ml", lines 244-299, characters 2-44:
# Error: This variant or record definition does not match that of type
#          Parsetree.pattern_desc
#        Constructors number 17 have different names, Ppat_effect and Ppat_extension.

[talex5]

I think this is fixed now. At least, it builds on my machine and in CI. If it doesn't work for you, try pulling a newer version of the base image.