Add Linux file uploads for the AppImage

CI Fixes

Implement CI uploading for Linux images (Windows and MacOS to follow later).

Uses Docker in the CI to install dependencies once.

Pins the build environment to use Node 14.17.3. See below for updating.

Partially addresses https://gitlab.com/nomadic-labs/umami-wallet/umami/-/issues/304

Docker workflow

After making a change to ci/Dockerfile, update .gitlab-ci.yml so that BUILD_CONTAINER_VERSION is incremented according to SemVer (or whatever standard you like). Run the manual pipeline that uploads the new container to registry, then re-run the failed build stage. Subsequent build pipelines using the same Docker image will run fine automatically, and there is no risk of using the wrong image accidentally, since the version is stored in a variable and gets updated everywhere at once.

Note: It is manual so that it doesn't clog up the CI building it on every commit. The unfortunate part of having to rerun the failed build stage is a work-around because the build stage can't wait for the manual step (since it usually won't be run anyways).