Update versions of test dependencies (test_requirements.txt and test containers).

docs/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.8.1-alpine3.11 as borgmatic
+FROM python:3.8-alpine3.12 as borgmatic
 
 COPY . /app
 RUN pip install --no-cache /app && generate-borgmatic-config && chmod +r /etc/borgmatic/config.yaml
@@ -7,7 +7,7 @@ RUN borgmatic --help > /command-line.txt \
            echo -e "\n--------------------------------------------------------------------------------\n" >> /command-line.txt \
            && borgmatic "$action" --help >> /command-line.txt; done
 
-FROM node:13.7.0-alpine as html
+FROM node:15.2.1-alpine as html
 
 ARG ENVIRONMENT=production
 
@@ -26,7 +26,7 @@ COPY . /source
 RUN NODE_ENV=${ENVIRONMENT} npx eleventy --input=/source/docs --output=/output/docs \
   && mv /output/docs/index.html /output/index.html
 
-FROM nginx:1.16.1-alpine
+FROM nginx:1.19.4-alpine
 
 COPY --from=html /output /usr/share/nginx/html
 COPY --from=borgmatic /etc/borgmatic/config.yaml /usr/share/nginx/html/docs/reference/config.yaml