perf(docker): optimize build for 3-core system

- Set MIX_OS_DEPS_COMPILE_PARTITION_COUNT to 3 (match available cores)
- Enable Erlang JIT perf improvements
- Remove redundant hex/rebar installation calls (already done earlier)

This won't eliminate Docker compile time but should make better use of
the 3 available cores. Estimated savings: 1-2 minutes on build.

Dockerfile

@@ -21,7 +21,10 @@ ARG BUILDER_IMAGE="docker.io/hexpm/elixir:${ELIXIR_VERSION}-erlang-${OTP_VERSION
 ARG RUNNER_IMAGE="docker.io/debian:${DEBIAN_VERSION}"
 
 FROM ${BUILDER_IMAGE} AS builder
-ENV MIX_OS_DEPS_COMPILE_PARTITION_COUNT=6
+ENV MIX_OS_DEPS_COMPILE_PARTITION_COUNT=3
+# Enable parallel compilation with 3 cores
+ENV ELIXIR_ERL_OPTIONS="+JPperf true"
+ENV ERL_AFLAGS="-kernel shell_history enabled"
 # Disable TTY for cross-architecture builds
 ENV TERM=dumb
 
@@ -46,8 +49,6 @@ COPY mix.exs mix.lock ./
 COPY vendor vendor
 RUN --mount=type=cache,target=/root/.hex \
     --mount=type=cache,target=/root/.mix \
-    mix local.hex --force && \
-    mix local.rebar --force && \
     mix deps.get --only $MIX_ENV
 RUN mkdir config
 
@@ -57,8 +58,6 @@ RUN mkdir config
 COPY config/config.exs config/${MIX_ENV}.exs config/
 RUN --mount=type=cache,target=/root/.hex \
     --mount=type=cache,target=/root/.mix \
-    mix local.hex --force && \
-    mix local.rebar --force && \
     mix deps.compile
 
 RUN --mount=type=cache,target=/root/.mix \