Not for a simple SQL adaptor using JGW my friend...

1. No, the postgresql is not in the docker-compose, but is easy to include:

version: '3.6'
services:
  db:
    image: postgres
    restart: always
    environment:
      POSTGRES_USER : demo
      POSTGRES_DB: demo
      POSTGRES_PASSWORD: demo
    container_name: demo-pg
    volumes:
      - ${PWD}/pg-scripts:/docker-entrypoint-initdb.d/
    ports:
      - 5432:5432

The Docker file is a bit messy because testing... sure you can clean it:

FROM openjdk:8 AS builder
ARG APP_HOME=/tmp/app

# copy JDBCTest
COPY src $APP_HOME/src

# copy JGW jars
COPY --from=intersystemsdc/jgw:latest /jgw/*.jar $APP_HOME/jgw/

# compile and create JDBCTest
WORKDIR $APP_HOME/src
RUN javac -classpath $APP_HOME/jar/*:$APP_HOME/jgw/* JDBCTest.java && \
    mkdir $APP_HOME/jar && \
    jar -cvf $APP_HOME/jar/JDBCTest.jar JDBCTest.class

WORKDIR $APP_HOME/jar/
ADD https://jdbc.postgresql.org/download/postgresql-42.2.18.jar .

FROM openjdk:8u252-jre

ENV GWDIR /jgw
ENV PORT 55555
# add LOG
ENV LOG /tmp/jgw-trace.log
ENV ISC_PACKAGE_INSTALLDIR /usr/irissys
ENV ISC_JARDIR $ISC_PACKAGE_INSTALLDIR/dev/java/lib/JDK18

COPY --from=store/intersystems/iris-community:2020.2.0.211.0 \
     $ISC_JARDIR/intersystems-gateway-3.1.0.jar \
     $ISC_JARDIR/intersystems-jdbc-3.1.0.jar \
     $ISC_JARDIR/intersystems-utils-3.1.0.jar \
     $ISC_PACKAGE_INSTALLDIR/dev/java/lib/gson/gson-2.8.5.jar \
     $GWDIR/

CMD java $JVMARGS -Xrs -classpath "$GWDIR/*" com.intersystems.gateway.JavaGateway $PORT $LOG 2>&1

COPY --from=builder /tmp/app/jar/*.jar $GWDIR/

## install additional packages for debug
RUN apt-get update && apt-get install -y procps

JDBCTest is a simple JDBC java class for testing the connection

I have manually set chmod +777 to jars but still getting the same error