Dockerfile: Update base Golang version. Add 'dave' group.

[pataquets]

Fixes #59. Supercedes #58.

• Updated Golang base image to latest (as of Oct'23: 1.21.3). I might pin it to just 1.21 to use whatever latest 1.21.x is, if desired.
• As done in #58, ensures the group dave with GID 1000 exists and explicitly adds user dave to it, resulting in 1000:1000 UID:GID. As done with the user, group is also added as system (-S).

Notice: keep an eye on micromata/dave Tags on Docker Hub to check if the Github Action builds successfully. The Docker image is quite outdated (eg. the --config switch doesn't work as per readme).

[chclaus]

Hej @pataquets,

thanks for your PR and thanks for the hint with the Docker Hub repo. We'll have a look to deploy a new version.

Best, Christian