Add jq to container image

[Skaronator]

What does this PR change? What problem does it solve?

Many commands in Restic already allow JSON as output (restic snapshots --json). Therefore, it would be very helpful if the official container image contains jq which can be very useful for scripts or simple inline commands.

One example I'd like to use would be this command: restic diff $(restic snapshots --json | jq -r '.[-2:][].id') It shows the diff from the last 2 snapshots.

Was the change previously discussed in an issue or on the forum?

Not that I know.

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[fd0]

Fine with me :)

[MichaelEischer]

Just for documentation: we'd like to keep the docker image rather minimal, but jq is probably worth an exception.