...
services:
  ...
  frontend:
    # image: nginx:alpine
    build: ./mini_nginx
    ...

# start from an existing image
FROM alpine

# run this command in the guest system (package installation)
RUN apk add --no-cache nginx envsubst

# create a working directory and execute every remaining command from it
WORKDIR /mini_nginx

# copy the specified file(s) from the host to the working directory
COPY mini_nginx.sh .

# run this command by default (from the working directory)
CMD ["/bin/sh", "mini_nginx.sh"]

#!/bin/sh
set -e # exit script on any failure

NGINX_PORT=${NGINX_PORT:-80}
NGINX_HTTPS_PORT=${NGINX_HTTPS_PORT:-443}
vars=`env | sed -re 's/(.*)=.*/${\1}/'`
mkdir -p /etc/nginx/templates
for f in /etc/nginx/templates/*.template ; do
  [ -f "${f}" ] || continue
  c='/etc/nginx/http.d/'`basename "${f}" .template`
  echo "generating ${c}"
  envsubst "${vars}" <"${f}" >"${c}"
done
echo "starting nginx server"
exec nginx -g 'daemon off;'

...
services:
  ...
  db:
    # image: postgres:alpine
    build: ./mini_postgres
    ...

# start from an existing image
FROM alpine

# run this command in the guest system (package installation)
RUN apk add --no-cache postgresql

# create a working directory and execute every remaining command from it
WORKDIR /mini_postgres

# copy the specified file(s) from the host to the working directory
COPY mini_postgres.sh .

# run this command by default (from the working directory)
CMD ["/bin/sh", "mini_postgres.sh"]

#!/bin/sh
set -e # exit script on any failure

missing=""
for n in POSTGRES_DB POSTGRES_USER POSTGRES_PASSWORD ; do
  eval v=\"\$${n}\"
  if [ -z "${v}" ] ; then
    echo "expected value for variable ${n}"
    missing="${missing} ${n}"
  fi
done
[ -z "${missing}" ] || exit 1

PG_MAJOR=`postgres -V | awk '{print $3}' | cut -d. -f1`
PGDATA=/var/lib/postgresql/${PG_MAJOR}/docker
for d in ${PGDATA} /run/postgresql ; do
  mkdir -p ${d}
  chown -R postgres:postgres ${d}
done
mkdir -p /docker-entrypoint-initdb.d
exec su postgres <<EOF
if [ ! -f ${PGDATA}/PG_VERSION ] ; then
  echo "initialising database"
  initdb -D ${PGDATA}
  echo "host all all all scram-sha-256" >>${PGDATA}/pg_hba.conf
  pg_ctl -D ${PGDATA} -o "-c listen_addresses='localhost'" -w start
  psql -c "CREATE USER ${POSTGRES_USER} WITH PASSWORD '${POSTGRES_PASSWORD}'"
  psql -c "CREATE DATABASE ${POSTGRES_DB} OWNER ${POSTGRES_USER};"
  for f in /docker-entrypoint-initdb.d/*.sql ; do
    [ -f "\${f}" ] || continue
    PGPASSWORD='${POSTGRES_PASSWORD}' psql -U ${POSTGRES_USER} \
      -d ${POSTGRES_DB} -v ON_ERROR_STOP=1 <"\${f}"
  done
  pg_ctl -D ${PGDATA} -m fast -w stop
fi
echo "starting postgres server"
exec postgres -D ${PGDATA} -c listen_addresses='*'
EOF

app_demo_2
├── ...
├── rs_backend
│   ├── Cargo.toml
│   ├── Dockerfile
│   └── src
│       └── main.rs
└── ...

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# builder image
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# start from an existing image
FROM rust:alpine AS builder

# create a working directory and execute every remaining command from it
WORKDIR /app

# copy the specified file(s) from the host to the working directory
COPY Cargo.toml .

# run this command in the guest system (dependency compilation in cache)
RUN --mount=type=cache,target=/app/target \
    mkdir src && echo 'fn main() {}' >src/main.rs && cargo build --release

# copy the specified file(s) from the host to the working directory
COPY src/ src/

# run this command in the guest system (program compilation)
RUN --mount=type=cache,target=/app/target \
    find src -exec touch {} + \
 && cargo build --release && cp target/release/rs_backend .

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# final image
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# start from an EMPTY image
FROM scratch

# copy the compiled program from the builder image
COPY --from=builder /app/rs_backend /

# run this command by default
ENTRYPOINT ["/rs_backend"]

...
services:
  ...
  backend:
    # build: ./py_backend
    build: ./rs_backend
    ...