feat(yamlfmt): Migrate to the "pattern file" discovery method.

This new feature in yamlfmt 0.15.0 allows us to specify which files to format
(and which to ignore) using a file with the "gitignore" syntax.

yamlfmt.md

@@ -20,7 +20,27 @@ include:
 The *yamlfmt* job provides very limited configuration options. This is by design.
-### Excluding files
-To exclude files, create a file named `.yamlfmtignore` at the root of your repository.
-The file uses [gitignore](https://git-scm.com/docs/gitignore) syntax to excempt some files, such as test data, from formatting.
+### Including and excluding files
+The `.yamlfmt.patterns` file allows to specify additional files and/or exclude files from the format check.
+It is sometimes desireable to check the format of files that do not have the usual `.yaml` or `.yml` suffix.
+At other times it is necessary to exclude files from the format check, for example intentionally malformed test data.
+
+To include or exclude files, create a file named `.yamlfmt.patterns` at the root of your repository.
+
+Files to **include** are added as normal (positive) entries, for example:
+
+```gitignore
+/.some-tool # dotfile using YAML syntax but without the suffix
+```
+
+Files to **exclude** are added a negated entries, for example:
+
+```gitignore
+!testdata/
+```
+
+The `.yamlfmt.patterns` file uses [gitignore](https://git-scm.com/docs/gitignore) syntax.
+Despite the name, only negated entries (beginning with an exclamation point `!`) are *excluded*, all other entries are *included*.
+
+The `.yamlfmt.patterns` file is combined with [the default `yamlfmt.patterns` file](https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks-images/-/blob/main/yamlfmt/yamlfmt.patterns?ref_type=heads) with the default file coming first and the `.yamlfmt.patterns` file second/below.

yamlfmt.yml

@@ -11,21 +11,19 @@ yamlfmt:
     name: registry.gitlab.com/gitlab-com/gl-infra/common-ci-tasks-images/yamlfmt:${GL_ASDF_YAMLFMT_VERSION}
     entrypoint: [""]
   script:
-    # yamlfmt will fail if this file doesn't exist.
-    - touch .yamlfmtignore
-    - |
-      declare -a PATTERNS=('*.yaml' '*.yml')
-      if [[ -e .yamlfmtextra ]]; then
-        IFS=' '
-        while read -r pattern; do
-          PATTERNS+=("${pattern}")
-        done <<<$(grep -E -v '^\s*(#|$)' .yamlfmtextra)
-      fi
     - |
       # Dump yamlfmt config
       echo "# /etc/yamlfmt.yaml"
       cat /etc/yamlfmt.yaml
-    - yamlfmt -conf /etc/yamlfmt.yaml "${PATTERNS[@]}"
+    - |
+      # Dump pattern file
+      PATTERN_FILES=("/etc/yamlfmt.patterns")
+      if [[ -e .yamlfmt.patterns ]]; then
+        PATTERN_FILES+=(".yamlfmt.patterns")
+      fi
+      echo "# ${PATTERN_FILES[@]}"
+      cat "${PATTERN_FILES[@]}"
+    - yamlfmt -conf /etc/yamlfmt.yaml "${PATTERN_FILES[@]}"
     - git diff | tee yamlfmt.patch
     - |
       if [[ -s yamlfmt.patch ]]; then