ci: grant contents write permission for SBOM release attachments (#7)

The Docker build workflow fails with "Resource not accessible by
integration" when attaching SBOMs to releases because the build job has
`contents: read` permission.

## Changes

- Changed `build` job's `permissions.contents` from `read` to `write` in
`.github/workflows/docker-build.yml`

```yaml
build:
  runs-on: ubuntu-latest
  permissions:
-   contents: read
+   contents: write
    packages: write
    id-token: write
```

This allows the workflow to attach SBOM artifacts to GitHub releases
when tags are pushed.

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> Update the workflow file .github/workflows/docker-build.yml in
repository bakerboy448/RedditModLog to fix the failing job that errors
with 'Resource not accessible by integration' when attaching SBOMs to a
release. The build job currently has permissions.contents: read which
prevents the workflow from attaching files to releases. Change the build
job's permissions.contents from 'read' to 'write' and make no other
changes. Create a new branch named fix/sbom-permissions, commit the
change with message 'ci: grant contents write permission so SBOMs can be
attached to releases', and open a pull request against the main branch
with the given title. Ensure the PR only modifies
.github/workflows/docker-build.yml and explain in the PR description why
the change is needed and how to verify it fixes the issue.


</details>

*This pull request was created as a result of the following prompt from
Copilot chat.*
> Update the workflow file .github/workflows/docker-build.yml in
repository bakerboy448/RedditModLog to fix the failing job that errors
with 'Resource not accessible by integration' when attaching SBOMs to a
release. The build job currently has permissions.contents: read which
prevents the workflow from attaching files to releases. Change the build
job's permissions.contents from 'read' to 'write' and make no other
changes. Create a new branch named fix/sbom-permissions, commit the
change with message 'ci: grant contents write permission so SBOMs can be
attached to releases', and open a pull request against the main branch
with the given title. Ensure the PR only modifies
.github/workflows/docker-build.yml and explain in the PR description why
the change is needed and how to verify it fixes the issue.

<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
tips](https://gh.io/copilot-coding-agent-tips) in the docs.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bakerboy448 <55419169+bakerboy448@users.noreply.github.com>

Author: Copilot

.github/workflows/docker-build.yml

@@ -30,7 +30,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     permissions:
-      contents: read
+      contents: write
       packages: write
       id-token: write
     outputs: