fix: cwltool.Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE313-APKTOOLS-1246345

Author: snyk-bot

cwltool.Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.6-alpine as builder
+FROM python:3.8-alpine as builder
 
 RUN apk add --no-cache git gcc python3-dev libxml2-dev libxslt-dev libc-dev linux-headers
 
@@ -9,13 +9,13 @@ RUN python setup.py bdist_wheel --dist-dir=/wheels
 RUN pip wheel -r requirements.txt --wheel-dir=/wheels
 RUN pip install --no-index --no-warn-script-location --root=/pythonroot/ /wheels/*.whl
 
-FROM python:3.6-alpine as module
+FROM python:3.8-alpine as module
 LABEL maintainer [email protected]
 
 RUN apk add --no-cache docker nodejs graphviz libxml2 libxslt
 COPY --from=builder /pythonroot/ /
 
-FROM python:3.6-alpine
+FROM python:3.8-alpine
 LABEL maintainer [email protected]
 
 RUN apk add --no-cache docker nodejs graphviz libxml2 libxslt