chore(release) install CMake >= 3.12 on Ubuntu 18.04

The latest cargo uses cmake --parallel when building
wabt-sys (which is required by cwabt, used by the V8 backend).
The --parallel flag is only available from CMake 3.12 onwards,
which is not included in the Ubuntu 18.04 distro.

More info on the cmake version requirement for cmake-rs:
rust-lang/cmake-rs#131
A fix was merged but a new version of the crate was not
yet released:
rust-lang/cmake-rs#146

Additionally, wabt-sys actually includes code which assumes
CMake >= 3.12! See:
https://ma.ttias.be/wabt-sys-compile-error-cmake-project-version-not-declared-in-scope-ubuntu-18-04/

assets/release/Dockerfiles/Dockerfile.amd64.ubuntu-18.04

@@ -16,6 +16,15 @@ RUN apt-get update && \
         clang \
         curl
 
+# Install CMake >= 3.12 (required by wabt-sys,
+# which is required by cwabt, used by the V8 backend).
+RUN apt-get install -y --no-install-recommends \
+        python3-pip \
+        python3-setuptools && \
+        pip3 install --upgrade pip && \
+        pip3 install scikit-build && \
+        pip3 install cmake
+
 ENV RUSTUP_HOME=/usr/local/rustup CARGO_HOME=/usr/local/cargo
 ENV PATH $CARGO_HOME/bin:$PATH
 RUN mkdir -p "$CARGO_HOME" && mkdir -p "$RUSTUP_HOME" && \