diff --git a/backend/Dockerfile b/backend/Dockerfile
index 48d3217..da46e92 100644
--- a/backend/Dockerfile
+++ b/backend/Dockerfile
@@ -3,7 +3,7 @@ FROM python:3.7-alpine AS build
 ARG EXTRA_DEPS
 RUN apk add build-base musl-dev libffi-dev openssl-dev mariadb-dev
 WORKDIR /app
-RUN pip install -U setuptools 'cryptography>=3.0,<3.1' poetry==1.1.7 ${EXTRA_DEPS}
+RUN pip install -U setuptools 'cryptography>=3.0,<3.1' poetry==1.1.7 $EXTRA_DEPS
 COPY pyproject.toml poetry.lock ./
 RUN poetry config virtualenvs.create false
 RUN poetry install --no-dev --no-ansi --no-interaction