feat: env-configurable JAVA_HOME / SPARK_HOME via entrypoint

Three problems with the prior Dockerfile:

  1. JAVA_HOME and SPARK_HOME were hardcoded at build time (ENV ...),
     so docker-compose / .env overrides had no effect.
  2. PATH was constructed at build time using those hardcoded values,
     so even if you could override the env vars, PATH would still
     reference the old literal paths (e.g. /usr/lib/jvm/java-17-openjdk-amd64/bin
     hardcoded into PATH at image build).
  3. There was no .env.example entry for either, so operators had no
     template to follow.

Fix:
  - docker-entrypoint.sh: re-derives PATH from the (possibly overridden)
    JAVA_HOME and SPARK_HOME at every container start. Strips any stale
    JDK/Spark bin dirs from PATH first so a restart with a new override
    actually changes which  /  resolve.
  - Dockerfile: COPY + ENTRYPOINT [entrypoint.sh], CMD [gunicorn main:app].
    The existing ENV JAVA_HOME and ENV SPARK_HOME stay as the defaults
    so the image works out of the box; users override via .env.
  - .env.example: new 'Java + Spark install paths' section, default
    values match the Dockerfile, comments explain the entrypoint
    re-derivation.
  - docker-compose.yml: forwards JAVA_HOME and SPARK_HOME with
    container-side defaults matching the Dockerfile.

Verified:
  - 116/116 tests still pass
  - Direct entrypoint run with JAVA_HOME=/fake/jdk shows PATH rebuilt as
    /app/.venv/bin:/fake/jdk/bin:/fake/spark/bin:... (override took effect)

Note: uses awk instead of 'paste -sd:' for portability across macOS
(BSD paste doesn't support -s) and Linux (GNU does).

.env.example

@@ -53,6 +53,20 @@ YARN_RESOURCE_MANAGER_URL=
 # Useful for proxies, custom truststores, or driver memory caps.
 # SPARK_SUBMIT_OPTS=-Dhttps.proxyHost=proxy.corp -Dhttps.proxyPort=3128
 
+# --- Java + Spark install paths ---
+# Where the openjdk-17-jre-headless JDK lives, and where the Spark
+# distribution was extracted during the image build. Defaults match the
+# Dockerfile's ENTRYPOINT script. Override if you mount a different
+# Java (e.g. /usr/lib/jvm/java-17-openjdk-arm64 on some ARM hosts) or
+# a pre-installed Spark from a host volume (e.g. /opt/spark-3.5.1-bin-hadoop3).
+#
+# The container's docker-entrypoint.sh re-derives PATH from these values
+# at every start, so overriding them here actually changes which `java`
+# and `spark-submit` binaries the gunicorn process picks up.
+#
+# JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64
+# SPARK_HOME=/opt/spark
+
 # --- Gunicorn process model (see gunicorn.conf.py; NOT read by common/config.py) ---
 # Defaults shown. These are read by gunicorn directly, not by the app.
 # GUNICORN_WORKERS=2