Fix release building of assets

It's a little bit more complicated than though in the previous commit:
- the Dockerfile will be run only when building for linux
- The script is run both for linux and mac
- The Dockerfile is run first

I think duplicating dune subst is ok to have both that it is run first in linux
and in mac modes.

Author: panglesd

release/Dockerfile

@@ -8,3 +8,7 @@ RUN opam install -y --deps-only --with-test --with-doc .
 
 COPY --chown=opam . .
 
+RUN dune subst
+
+RUN sed -i 's/"()"/"(-cclib -static -cclib -no-pie)"/g' src/compiler/bin/static-linking-flags/static_linking_flags.ml
+

release/release.sh

@@ -5,8 +5,6 @@ archive_name=$OUTPUT/slipshow-$TARGETOS-$TARGETARCH.tar
 
 dune subst
 
-sed -i 's/"()"/"(-cclib -static -cclib -no-pie)"/g' src/compiler/bin/static-linking-flags/static_linking_flags.ml
-
 dune build --profile release -p slipshow
 
 # Executables are symlinks, follow with -h.