Split plv8 build into two parts (#10920)

Plv8 consists of two parts:
1. the V8 engine, which is built from vendored sources, and
2. the PostgreSQL extension.

Split those into two separate steps in the Dockerfile. The first step
doesn't need any PostgreSQL sources or any other files from the neon
repository, just the build tools and the upstream plv8 sources. Use the
build-deps image as the base for that step, so that the layer can be
cached and doesn't need to be rebuilt every time. This is worthwhile
because the V8 build takes a very long time.

compute/compute-node.Dockerfile

@@ -395,15 +395,22 @@ RUN case "${PG_VERSION:?}" in \
     cd plv8-src && \
     if [[ "${PG_VERSION:?}" < "v17" ]]; then patch -p1 < /ext-src/plv8-3.1.10.patch; fi
 
-FROM pg-build AS plv8-build
+# Step 1: Build the vendored V8 engine. It doesn't depend on PostgreSQL, so use
+# 'build-deps' as the base. This enables caching and avoids unnecessary rebuilds.
+# (The V8 engine takes a very long time to build)
+FROM build-deps AS plv8-build
 ARG PG_VERSION
+WORKDIR /ext-src/plv8-src
 RUN apt update && \
     apt install --no-install-recommends --no-install-suggests -y \
     ninja-build python3-dev libncurses5 binutils clang \
     && apt clean && rm -rf /var/lib/apt/lists/*
-
 COPY --from=plv8-src /ext-src/ /ext-src/
-WORKDIR /ext-src/plv8-src
+RUN make DOCKER=1 -j $(getconf _NPROCESSORS_ONLN) v8
+
+# Step 2: Build the PostgreSQL-dependent parts
+COPY --from=pg-build /usr/local/pgsql /usr/local/pgsql
+ENV PATH="/usr/local/pgsql/bin:$PATH"
 RUN \
     # generate and copy upgrade scripts
     make generate_upgrades && \