.github: bump actions/checkout from 3.6.0 to 4.0.0 (#76)

See the release page [1] and new commits [2].

[1] https://github.com/actions/checkout/releases/tag/v4.0.0
[2] actions/checkout@f43a0e5...3df4ab1

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout code
-        uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744
+        uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@4c0219f9ac95b02789c1075625400b2acbff50b1

.github/workflows/lint_dockerfile.yml

@@ -7,7 +7,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744
+        uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac
 
       - name: Run hadolint
         uses: hadolint/hadolint-action@54c9adbab1582c2ef04b2016b760714a4bfde3cf

.github/workflows/shellcheck.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout
-        uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744
+        uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac
 
       - name: Run shellcheck
         uses: ludeeus/action-shellcheck@00cae500b08a931fb5698e11e79bfbd38e612a38