Compose: Impossível COPIAR via docker-compose

Tendo a seguinte estrutura de projeto:

project_root/.travis/debian-stretch/minion/Dockerfile

FROM debian:stretch
COPY .travis/entrypoint_minion_run.sh /opt/
COPY .travis/config/minion.conf /etc/salt/minion.d/minion.conf
COPY .travis/config/supervisord.conf /etc/supervisor/conf.d/supervisord.conf
RUN apt-get update && \
    apt-get install -y curl supervisor && \
    curl -L https://bootstrap.saltstack.com | sh

ENTRYPOINT ["/opt/entrypoint_minion_run.sh"]

project_root/.travis/debian-stretch/docker-compose.yml

version: '3.4'
services:
  master:
    build: .travis/debian-stretch/master
    hostname: master
    domainname: public.com
    ports:
     - "4505:4505"
     - "4506:4506"
    links:
      - minion1
      - minion2
      - minion3
  minion1:
    build: .travis/debian-stretch/minion
    hostname: minion1
    domainname: public.com
  minion2:
    build: .travis/debian-stretch/minion
    hostname: minion2
    domainname: public.com
  minion3:
    build: .travis/debian-stretch/minion
    hostname: minion3
    domainname: public.com

De project_root :

• Posso executar compilações de docker build -f .travis/debian-stretch/minion/Dockerfile . container bem
• Não consigo correr: docker-compose -f .travis/debian-stretch/docker-compose.yml --project-directory=. build minion1

Recebo a seguinte falha:

Building minion1
Step 1/6 : FROM debian:stretch
 ---> 6d83de432e98
Step 2/6 : COPY .travis/entrypoint_minion_run.sh /opt/
ERROR: Service 'minion1' failed to build: COPY failed: stat /var/lib/docker/tmp/docker-builder127631070/.travis/entrypoint_minion_run.sh: no such file or directory

Claro que entrypoint_minion_run.sh existe sob project_root/.travis/entrypoint_minion_run.sh

É um bug ou algo errado com meu docker-compose.yml ou minha configuração?

Minha versão docker-ce: Docker version 17.09.0-ce, build afdb6d4
Minha versão docker-compose: docker-compose version 1.17.1, build 6d101fb