Redo CI to run on armv6

I've refactored the CI substantially, using the
new `extends` keyword to tidy it up a lot.
I have also changed from running the Python
tests on Python:3.7 to using a Raspberry Pi
OS from the Balena project.

This means we are now ONLY locking
dependencies for the Pi, and we are no longer
expecting the Pipfile to work on non-arm
systems.

.gitlab-ci.yml

@@ -6,8 +6,12 @@ stages:
   - deploy
 
 # Re-usable block to install (and cache) Poetry and openflexure-microscope-server
-.python-install-template: &python-install
+.python:
+  image: balenalib/raspberry-pi:buster
+  retry: 1
   before_script:
+    - apt-get update
+    - apt-get install python3 python3-pip python3-venv
     - python3 -m venv .venv        # NB this does not overwrite an existing cached venv
     - source .venv/bin/activate
     - pip install pipenv           # Should be cached after first run
@@ -17,9 +21,14 @@ stages:
     paths:
       - .cache/pip
       - .venv
+  only:
+    - branches
+    - merge_requests
+    - tags
+    - web
 
-# Re-usable block to install (and cache) openflexure-microscope-server static app
-.node-install-template: &node-install
+.javascript:
+  image: node:15
   before_script:
     - cd webapp
     - npm install
@@ -27,95 +36,54 @@ stages:
     key: "${CI_COMMIT_REF_SLUG}"
     paths:
       - webapp/node_modules
-
-# Python static analysis with PyLint
-pylint:
-  stage: analysis
-  image: python:3.7
-  retry: 1
-
-  <<: *python-install
-
-  script:
-    - poe pylint
-
   only:
     - branches
     - merge_requests
     - tags
     - web
 
+# Python static analysis with PyLint
+pylint:
+  stage: analysis
+  extends: .python
+  script:
+    - poe pylint
+
 # Python type checking with Mypy
 mypy:
   stage: analysis
-  image: python:3.7
-  retry: 1
-
-  <<: *python-install
-
+  extends: .python
   script:
     - poe mypy
-
   artifacts:
     reports:
       coverage_report: 
         coverage_format: cobertura
         path: openflexure_microscope/cobertura.xml
 
-  only:
-    - branches
-    - merge_requests
-    - tags
-    - web
-
 # Python style analysis with Black
 black:
   stage: analysis
-  image: python:3.7
-  retry: 1
   allow_failure: true
-
-  <<: *python-install
-
+  extends: .python
   script:
-    # Run static build script
     - poe black_check
 
-  only:
-    - branches
-    - merge_requests
-    - tags
-    - web
-
 # Python unit tests with PyTest
 pytest:
   stage: testing
-  image: python:3.7
-  retry: 1
-
-  <<: *python-install
-
+  extends: .python
   script:
     - poe test
-
-  only:
-    - branches
-    - merge_requests
-    - tags
-    - web
-
   artifacts:
     when: always
     reports:
       junit: pytest_report.xml
 
+# Generate and validate OpenAPI description
 openapi:
   stage: testing
-  image: python:3.7
-  retry: 1
-
-  <<: *python-install
-
+  extends: .python
   script:
     - mkdir -p docs/build/
     - ofm-generate-openapi -o docs/build/swagger.yaml --validate
@@ -126,52 +94,27 @@ openapi:
     paths:
       - "./docs/build/swagger.yaml"
 
-  only:
-    - branches
-    - merge_requests
-    - tags
-    - web
-
 # JavaScript linting with ESLint (via Vue CLI)
 eslint:
   stage: analysis
-  image: node:15
-
-  <<: *node-install
-
+  extends: .javascript
   script:
     # Build JS application for production
     - npm run lint
 
-  only:
-    - branches
-    - merge_requests
-    - tags
-    - web
-
 # Build JS app
 build:
   stage: build
-  image: node:15
-
-  <<: *node-install
-
+  extends: .javascript
   script:
     # Build JS application for production
     - npm run build
-
   artifacts:
     name: "dist"
     expire_in: 1 week
     paths:
       - "openflexure_microscope/api/static/dist/"
 
-  only:
-    - master
-    - merge_requests
-    - tags
-    - web
-
 # Package application into distribution tarball
 package:
   stage: package