[Dev] remove unecessary docker commands and update README for custom docker

devel_tools/docker/frama-c.20.0/Dockerfile

 FROM debian:sid as base
 
+# Install non-OCaml dependencies + opam
 RUN apt-get update && apt-get install -y \
     cvc4 \
     opam \

devel_tools/docker/frama-c.21.0/Dockerfile

 FROM debian:sid as base
 
+# Install non-OCaml dependencies + opam
 RUN apt-get update && apt-get install -y \
     cvc4 \
     opam \
@@ -32,9 +33,6 @@ RUN apt-get update && opam update -y && opam depext --install -y --verbose \
     zmq.5.1.3 \
     && rm -rf /var/lib/apt/lists/*
 
-# Install non-OCaml solvers
-RUN apt install z3 cvc4 -y
-
 RUN why3 config --full-config
 
 # with_source: keep Frama-C sources

devel_tools/docker/frama-c.21.1/Dockerfile

 FROM debian:sid as base
 
+# Install non-OCaml dependencies + opam
 RUN apt-get update && apt-get install -y \
     cvc4 \
     opam \
@@ -32,9 +33,6 @@ RUN apt-get update && opam update -y && opam depext --install -y --verbose \
     zmq.5.1.3 \
     && rm -rf /var/lib/apt/lists/*
 
-# Install non-OCaml solvers
-RUN apt install z3 cvc4 -y
-
 RUN why3 config --full-config
 
 # with_source: keep Frama-C sources

devel_tools/docker/frama-c.custom/Dockerfile

 FROM debian:sid as base
 
+# Install non-OCaml dependencies + opam
 RUN apt-get update && apt-get install -y \
     cvc4 \
     opam \
@@ -32,9 +33,6 @@ RUN apt-get update && opam update -y && opam depext --install -y --verbose \
     zmq.5.1.3 \
     && rm -rf /var/lib/apt/lists/*
 
-# Install non-OCaml solvers
-RUN apt install z3 cvc4 -y
-
 RUN why3 config --full-config
 
 # with_source: keep Frama-C sources

devel_tools/docker/frama-c.custom/README.md

@@ -12,7 +12,8 @@ a custom git repository, do the following:
    `frama-c`.
 
 2. If needed, add Debian and opam prerequisites to the appropriate lines in the
-   Dockerfile, or change the opam version in the `ENV` lines.
+   Dockerfile (after `apt update` and before removing the `apt` cache).
+   If you need to change the opam version, also edit the `ENV` lines.
 
 3. For a "minimal" image (with installed Frama-C, but no source code), run:
 

devel_tools/docker/frama-c.dev/Dockerfile

 FROM debian:sid as base
 
+# Install non-OCaml dependencies + opam
 RUN apt-get update && apt-get install -y \
     cvc4 \
     opam \
@@ -32,9 +33,6 @@ RUN apt-get update && opam update -y && opam depext --install -y --verbose \
     zmq.5.1.3 \
     && rm -rf /var/lib/apt/lists/*
 
-# Install non-OCaml solvers
-RUN apt install z3 cvc4 -y
-
 RUN why3 config --full-config
 
 # with_source: keep Frama-C sources