Fix docker workdir mount and unknown output values

[sebastianrath]

This PR fixes #26 and #27.

For #26, the error occurred because an action can set an output that is not officially listed in the action.yml of a GH action.

https://github.com/getsentry/action-release/blob/e769183448303de84c5a06aaaddf9da7be26d6c7/src/main.ts#L83-L84

    core.debug(`Done`);
    core.setOutput('version', version);
  } catch (error) {

For #27, instead of the GitHub workspace the runner workspace was loaded, mounting the wrong dir.

Besides the reported issue, two more changes are introduced in this PR:

• Custom env vars are not propagated to GH container actions
• Load .env always next to the binary. Before it was loaded by the current working directory.