specify tagged lalsuite version of gstlal-dev in CI pipeline

.gitlab-ci.Dockerfile.rl8

-FROM containers.ligo.org/gstlal/gstlal-dev:lalsuite-master-x86_64
+ARG lalsuite_version_tag
+FROM containers.ligo.org/gstlal/gstlal-dev:$lalsuite_version_tag
 
 # Labeling/packaging stuff:
 LABEL name="GstLAL Runtime Package, RL8" \

.gitlab-ci.yml

 image: docker:latest
 
 variables:
+
+  # Set the gstlal-dev version tag, which corresponds to a tagged 
+  # version of lalsuite. A list of available tags is available in the
+  # gstlal-dev container registry:
+  #
+  # https://git.ligo.org/gstlal/gstlal-dev/container_registry/809
+  LALSUITE_VERSION_TAG: "lalsuite-master-x86_64"
+
+  # Docker pipeline variables:
   DOCKER_DRIVER: overlay
   DOCKER_BRANCH: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME
   DOCKER_LATEST: $CI_REGISTRY_IMAGE:latest
@@ -12,6 +21,7 @@ variables:
   # don't need git history
   GIT_DEPTH: 1
 
+  # GstLAL pipeline-specific runtime variables. 
   TMPDIR: /tmp
   GSTLAL_FIR_WHITEN: 0
 
@@ -46,7 +56,7 @@ stages:
 
 .levelN:rpm: &levelN-rpm-package
   interruptible: true
-  image: containers.ligo.org/gstlal/gstlal-dev:lalsuite-master-x86_64
+  image: containers.ligo.org/gstlal/gstlal-dev:${LALSUITE_VERSION_TAG}
   variables:
     GIT_STRATEGY: fetch
     RPM_BUILD_CPUS: 4
@@ -115,7 +125,7 @@ level2:rpm:gstlal-burst:
 
     # Build the container:
     - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY
-    - docker build --pull -t $DOCKER_BRANCH --file .gitlab-ci.Dockerfile.rl8 .
+    - docker build --pull -t $DOCKER_BRANCH --build-arg lalsuite_version_tag="${LALSUITE_VERSION_TAG}" --file .gitlab-ci.Dockerfile.rl8 .
     - docker push $DOCKER_BRANCH
   only:
     - schedules
@@ -183,7 +193,7 @@ latest_image:
 # test stages (rl8)
 test:gstlal:rl8:
   interruptible: true
-  image: containers.ligo.org/gstlal/gstlal-dev:lalsuite-master-x86_64
+  image: containers.ligo.org/gstlal/gstlal-dev:${LALSUITE_VERSION_TAG}
   stage: test-gstlal
   needs:
     - level0:rpm:gstlal
@@ -205,7 +215,7 @@ test:gstlal:rl8:
 
 test:gstlal-full-build:rl8:
   interruptible: true
-  image: containers.ligo.org/gstlal/gstlal-dev:lalsuite-master-x86_64
+  image: containers.ligo.org/gstlal/gstlal-dev:${LALSUITE_VERSION_TAG}
   stage: test-gstlal-full-build
   needs:
     - level0:rpm:gstlal
@@ -226,7 +236,7 @@ test:gstlal-full-build:rl8:
 
 test:gstlal-inspiral:rl8:
   interruptible: true
-  image: containers.ligo.org/gstlal/gstlal-dev:lalsuite-master-x86_64
+  image: containers.ligo.org/gstlal/gstlal-dev:${LALSUITE_VERSION_TAG}
   stage: test-inspiral
   needs:
     - level0:rpm:gstlal
@@ -246,7 +256,7 @@ test:gstlal-inspiral:rl8:
 
 test:gstlal-ugly:rl8:
   interruptible: true
-  image: containers.ligo.org/gstlal/gstlal-dev:lalsuite-master-x86_64
+  image: containers.ligo.org/gstlal/gstlal-dev:${LALSUITE_VERSION_TAG}
   stage: test-gstlal-ugly
   needs:
     - level0:rpm:gstlal
@@ -266,7 +276,7 @@ test:gstlal-ugly:rl8:
 
 test:gstlal-burst:rl8:
   interruptible: true
-  image: containers.ligo.org/gstlal/gstlal-dev:lalsuite-master-x86_64
+  image: containers.ligo.org/gstlal/gstlal-dev:${LALSUITE_VERSION_TAG}
   stage: test-burst
   needs:
     - level0:rpm:gstlal
@@ -285,7 +295,7 @@ test:gstlal-burst:rl8:
 
 test:offline:rl8:
   interruptible: true
-  image: containers.ligo.org/gstlal/gstlal-dev:lalsuite-master-x86_64
+  image: containers.ligo.org/gstlal/gstlal-dev:${LALSUITE_VERSION_TAG}
   stage: test-offline
   needs:
     - level0:rpm:gstlal