Build Stages:
- build-env     Create build environment image to build Untrunc,
- construction  Create image with a built Untrunc,
- production    Create Docker executable "docker-untrunc".

AV Libraries:
- dev           OS development packages,
- dev-headers   OS development packages with internal header files,
- libav-<vers>  Libav  version <vers> build from source ("libav-12.3"),
- ffmpeg-<vers> FFmpeg version <vers> build from source (like: "ffmpeg-4.0.1"),
- libav         Libav  build from sources from git master,
- ffmpeg        FFmpeg build from sources from git master,
- ""            Dockerfile default.

AV Library Configuration (when building from source):
- --enable-*    Libav/FFmpeg configuration options,
- ""            Dockerfile default.

The Site to get the AV Library sources from:
- origin        The original site of the AV library's project,
- github        The mirror on GitHub of the the AV library's project,
- ""            Dockerfile default.

Notes:
- Tries to build a small Docker executable.
- Uses Untrunc sources from git master, if no Untrunc sources are provided.

Build results for Untrunc production image:
- ubuntu parent image           :  81 MiB,
- dev / dev-headers             : 264 MiB,  4:15 min,
- libav-12.3 only, no extra libs:  96 MiB,  5:15 min,
- libav-12.3 + 3GP,MP3,H264,H265: 108 MiB,  6:15 min,
- ffmpeg-4.0.1  + all extra libs: 131 MiB, 10:30 min.