RAP-349 Update Docker container upon request

Adds a new build variable to force an update of the Docker container.
This variable can be set in a scheduled job to update the Docker image
periodically.
This way the Prefactor pipeline will keep using the latests versions of
its underlying software packages.

.gitlab-ci.yml

@@ -12,6 +12,7 @@ variables:
   CALIBRATOR_HBA_RESULTS_NAME: "results_calibrator.tar.gz"
   TARGET_HBA_RESULTS_NAME: "results_target.tar.gz"
   PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
+  BUILD_DOCKER_IMAGE: "0"
 
 stages:
   - versioning
@@ -58,7 +59,7 @@ build_base:
   extends: .prepare
   script:
     - |
-      if ! docker manifest inspect $INTEGRATION_BASE_IMAGE > /dev/null; then
+      if ! docker manifest inspect $INTEGRATION_BASE_IMAGE > /dev/null || [ "$BUILD_DOCKER_IMAGE" = "1" ]; then
         docker build --tag $INTEGRATION_BASE_IMAGE -f Docker/Dockerfile-base .
         docker push $INTEGRATION_BASE_IMAGE
       fi

Docker/Dockerfile-base

@@ -3,13 +3,6 @@ FROM ubuntu:20.04 as builder
 # This Docker image builds the dependencies for the Rapthor pipeline.
 # It lives on the head of its dependencies.
 
-# The Docker image is cached and not updated on every build.
-# When this file changes it forces an update of the Docker image.
-# Increment this value to get a fresh build.
-# Note decrementing may give an older cached value, therefore only
-# increment the value.
-ENV DOCKER_BUILD_FORCE_GENERATION=0
-
 # Install all build-time dependencies
 RUN export DEBIAN_FRONTEND=noninteractive && \ 
     apt-get update && \