chore(deps): pin dependencies (#18)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| koalaman/shellcheck-alpine | pinDigest |  -> `7b0682c` |
| [woodpeckerci/plugin-docker-buildx](https://codeberg.org/woodpecker-plugins/docker-buildx) ([source](https://codeberg.org/woodpecker-plugins/docker-buildx.git)) | pinDigest |  -> `c7ac0b0` |

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Reviewed-on: #18
Co-authored-by: Renovate Bot <renovate@mgrote.net>
Co-committed-by: Renovate Bot <renovate@mgrote.net>

.woodpecker/build.yml

@@ -7,7 +7,7 @@ variables: # https://woodpecker-ci.org/docs/usage/advanced-usage
 
 steps:
   selfhosted_push:
-    image: "woodpeckerci/plugin-docker-buildx:3.0.1"
+    image: "woodpeckerci/plugin-docker-buildx:3.0.1@sha256:c7ac0b01243466bef93863591940ce5eacf59759e9045433a6fc28d20c696d01"
     settings:
       dockerfile: Dockerfile
       repo: registry.mgrote.net/postfix

.woodpecker/lint.yml

@@ -19,7 +19,7 @@ steps:
           - tag
 
   shellcheck:
-    image: "koalaman/shellcheck-alpine:latest"
+    image: "koalaman/shellcheck-alpine:latest@sha256:7b0682c5786633fc075f69852e5c7521767e8dcb512c43cf8041b022bdc98198"
     commands:
       - |
         find . -type f -not -path './.git/*' -not -path './collections/*' -exec file {} \; | while IFS= read -r line; do