bug: update parent Docker image, use Python 3.8

Update the Dockerfile to install taskcat from python:3.8.13-alpine3.15,
after the taskcat/taskcat:latest image was rebuilt and now uses Alpine
v3.15 and Python 3.10.

In Python 3.3, abstract base classes were moved from the "collections"
module to "collections.abc". In Python 3.10, the classes were removed
from the "collections" module, which causes certain taskcat commands to
return with error "AttributeError module 'collections' has no attribute
'Mapping'". See https://bit.ly/3vnAnOr for more details.

Associated issue: #298

Dockerfile

@@ -1,11 +1,8 @@
-# For v0.9 and higher, the taskcat containers do not pin specific
-# versions. Rather, they always fetch the latest version from pip. See
-# https://dockr.ly/2BjpG7C to see the taskcat Dockerfile.
+FROM python:3.8.13-alpine3.15
 
-# hadolint disable=DL3007
-FROM taskcat/taskcat:latest
+RUN apk add --no-cache python3-dev~3.9 gcc~10 libc-dev~0.7 nodejs~16 npm~8 && rm -rf /var/cache/apk/*
 
-RUN apk add --no-cache nodejs~=12 npm~=12 && rm -rf /var/cache/apk/*
+RUN pip3 install taskcat==0.9.30 --upgrade
 
 COPY entrypoint.sh /entrypoint.sh
 RUN chmod +x /entrypoint.sh