Resolve "Customizable Ansible version"

Closes #3 (closed)

Dockerfile

 FROM registry.cyberbrain.pw/docker/alpine:latest AS builder
+ARG ANSIBLE_VERSION
 ENV PATH="/opt/ansible/bin:$PATH"
 RUN apk --no-cache add python3 && \
     apk --no-cache add --virtual build-dependencies \
@@ -14,8 +15,11 @@ RUN apk --no-cache add python3 && \
         setuptools \
         wheel \
         && \
+    ( test -z $ANSIBLE_VERSION && \
+        pip install --ignore-installed --no-cache-dir ansible || \
+        pip install --ignore-installed --no-cache-dir ansible\<=$ANSIBLE_VERSION \
+        ) && \
     pip3 install --ignore-installed --no-cache-dir \
-        ansible \
         ansible-lint \
         mitogen \
         jmespath \
@@ -35,6 +39,8 @@ RUN apk --no-cache add \
         sshpass \
         openssh-client \
         rsync \
-        git
+        git \
+        && \
+    rm -rf /var/cache/apk/*
 COPY --from=builder /opt/ansible /opt/ansible
 CMD [ "ansible", "--version" ]