build: harden ci.yaml permissions

Signed-off-by: Alex <aleksandrosansan@gmail.com>

Author: sashashura

.github/workflows/ci.yaml

@@ -30,6 +30,9 @@ env:
 # `java` (we could use the `_JAVA_OPTIONS` environment variable, but this prints
 # text on stderr and so can break tests which check the output of a program).
 
+permissions:
+  contents: read # to fetch code (actions/checkout)
+
 jobs:
   test_non_bootstrapped:
     runs-on: [self-hosted, Linux]
@@ -528,6 +531,9 @@ jobs:
           ./project/scripts/sbtPublish ";project scala3-bootstrapped ;publishSigned ;sonatypeBundleRelease"
 
   nightly_documentation:
+    permissions:
+      contents: write # to push pages branch (peaceiris/actions-gh-pages)
+
     runs-on: [self-hosted, Linux]
     container:
       image: lampepfl/dotty:2021-03-22
@@ -706,6 +712,10 @@ jobs:
 
 
   open_issue_on_failure:
+    permissions:
+      contents: read # to fetch code (actions/checkout)
+      issues: write # to create new issues (jasonetco/create-an-issue)
+
     runs-on: [self-hosted, Linux]
     container:
       image: lampepfl/dotty:2021-03-22