diff --git a/.hadolint.yaml b/.hadolint.yaml
index a29f1e0e..5f550fef 100644
--- a/.hadolint.yaml
+++ b/.hadolint.yaml
@@ -1,3 +1,4 @@
+---
 ignored:
     - DL3006
     - DL3008
diff --git a/.readthedocs.yml b/.readthedocs.yml
index 3a97aea2..47c269bf 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -1,3 +1,4 @@
+---
 # .readthedocs.yml
 # Read the Docs configuration file
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
diff --git a/.travis.yml b/.travis.yml
index 763376d7..5c411395 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,46 +1,48 @@
+---
 dist: bionic
 language: python
 python:
-    - 3.7
+  - 3.7
 sudo: required
 services:
-    - docker
+  - docker
 
 jobs:
-  include:    
+  include:
     - stage: diff-test
       install:
-      - pip install --upgrade pip
-      - make dev-env
-      - make lint-install
+        - pip install --upgrade pip
+        - make dev-env
+        - make lint-install
       script:
-      - set -e
-      - if [ $(make n-docs-diff) -ne 0 ]; then make docs; fi;
-      - if [ $(make n-other-diff) -ne 0 ]; then make lint-build-test-all DARGS="--build-arg TEST_ONLY_BUILD=1"; fi;
+        - set -e
+        - if [ $(make n-docs-diff) -ne 0 ]; then make docs; fi;
+        - if [ $(make n-other-diff) -ne 0 ]; then make lint-build-test-all DARGS="--build-arg TEST_ONLY_BUILD=1"; fi;
     - stage: push-tx
       install:
-      - pip install --upgrade pip
-      - make dev-env
-      script: 
-      - if [ $(make n-docs-diff DIFF_RANGE=$TRAVIS_COMMIT_RANGE) -ne 0 ]; then make tx-en; fi;
+        - pip install --upgrade pip
+        - make dev-env
+      script:
+        - if [ $(make n-docs-diff DIFF_RANGE=$TRAVIS_COMMIT_RANGE) -ne 0 ]; then make tx-en; fi;
     - stage: full-test
       install:
-      - pip install --upgrade pip
-      - make dev-env
-      - make lint-install
+        - pip install --upgrade pip
+        - make dev-env
+        - make lint-install
       script:
-      - set -e
-      - make docs
-      - make lint-build-test-all DARGS="--build-arg TEST_ONLY_BUILD=1"
+        - set -e
+        - make docs
+        - make lint-build-test-all DARGS="--build-arg TEST_ONLY_BUILD=1"
 
 stages:
-    - name: diff-test
-      if: type = pull_request
-    - name: push-tx
-      if: type = push AND branch = master
-    - name: full-test
-      if: type = cron AND branch = master
+  - name: diff-test
+    if: type = pull_request
+  - name: push-tx
+    if: type = push AND branch = master
+  - name: full-test
+    if: type = cron AND branch = master
 
 env:
   global:
+    # yamllint disable-line
     secure: 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
diff --git a/examples/docker-compose/notebook/letsencrypt-notebook.yml b/examples/docker-compose/notebook/letsencrypt-notebook.yml
index 8e918f61..1b7449f7 100644
--- a/examples/docker-compose/notebook/letsencrypt-notebook.yml
+++ b/examples/docker-compose/notebook/letsencrypt-notebook.yml
@@ -1,3 +1,4 @@
+---
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 
@@ -9,8 +10,8 @@ services:
     image: my-notebook
     container_name: ${NAME}
     volumes:
-     - "work:/home/jovyan/work"
-     - "secrets:/etc/letsencrypt"
+      - "work:/home/jovyan/work"
+      - "secrets:/etc/letsencrypt"
     ports:
       - "${PORT}:8888"
     environment:
diff --git a/examples/docker-compose/notebook/notebook.yml b/examples/docker-compose/notebook/notebook.yml
index 52bcb2ee..72002a5d 100644
--- a/examples/docker-compose/notebook/notebook.yml
+++ b/examples/docker-compose/notebook/notebook.yml
@@ -1,3 +1,4 @@
+---
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 
@@ -9,7 +10,7 @@ services:
     image: my-notebook
     container_name: ${NAME}
     volumes:
-     - "work:/home/jovyan/work"
+      - "work:/home/jovyan/work"
     ports:
       - "${PORT}:8888"
 
diff --git a/examples/docker-compose/notebook/secure-notebook.yml b/examples/docker-compose/notebook/secure-notebook.yml
index 5c4e53a4..6b38f9c2 100644
--- a/examples/docker-compose/notebook/secure-notebook.yml
+++ b/examples/docker-compose/notebook/secure-notebook.yml
@@ -1,3 +1,4 @@
+---
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 
@@ -9,7 +10,7 @@ services:
     image: my-notebook
     container_name: ${NAME}
     volumes:
-     - "work:/home/jovyan/work"
+      - "work:/home/jovyan/work"
     ports:
       - "${PORT}:8888"
     environment: