fix: switch to slim docker image

Dockerfile

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-FROM python:3.7
+FROM python:3.7-slim
 
 COPY dist/*.whl /tmp
 RUN pip3.7 install $(ls /tmp/*.whl); rm -f /tmp/*.whl