From the [CrossBuildPackagingGuidelines wiki page](https://wiki.debian.org/CrossBuildPackagingGuidelines) `DEB_BUILD_GNU_TYPE` is the architecture of the machine *building* the package, whereas `DEB_HOST_GNU_TYPE` is the architecture for which we are building the package. I noticed this issue while cross-compiling an ARM container on my Mac.
This uses `dpkg-architecture` to get the proper GNU tuple value for passing to `--build` on `./configure` invocations (which some arches can work without, but doing things like building 32bit images on 64bit kernels will break).
For some reason, versions 18 and 19 do not build successfully on ppc64le, so I've excluded those combinations explicitly in `generate-stackbrew-library.sh`.
This also includes the changes necessary to support the Alpine variants with proper multiarch once the upstream image is officially built that way (and the results of that for each architecture are also included below using the existing hacked-together Alpine images).
Here's the results from build testing this on each architecture:
`amd64`:
```
TAG IMAGE ID SIZE
17 1dd549b7ffa3 748MB
17-slim 6fbc2145c5f3 281MB
18 7e3e5c4a9f20 754MB
18-slim 5df9e72a4ef2 284MB
19 24ba3cb6c065 821MB
19-slim 1ad3a1c97322 375MB
20 24d24bceae50 874MB
20-alpine 297db53a7f3b 70.2MB
20-slim ff9f8003c780 277MB
master 802a5a8bb530 809MB
master-alpine a53503afc07e 68.2MB
```
`arm32v7`:
```
TAG IMAGE ID SIZE
17 df60315b83a1 616MB
17-slim 5a8c8979ba0e 242MB
18 c39107e65fb2 621MB
18-slim af17963b4999 245MB
19 b6af577ba752 674MB
19-slim 8eca11cda0c3 319MB
20 d821b51ff4dc 728MB
20-slim b1d2883c690e 240MB
master 1146f759e4ab 665MB
```
`arm64v8`:
```
TAG IMAGE ID SIZE
17 08c6e606c334 692MB
17-slim 24fc927a7eb3 269MB
18 c120c119bd84 698MB
18-slim 4f5d59a1a13f 272MB
19 60cc4c096f79 764MB
19-slim 86329f73c8e9 359MB
20 1a000e5163d7 817MB
20-alpine f747d5abe1d7 65.6MB
20-slim a77cbccb4422 265MB
master 1f1697a61c72 752MB
master-alpine 097e8165db40 63.6MB
```
`i386`:
```
TAG IMAGE ID SIZE
17 fac39a4759d0 734 MB
17-slim 83b2b2655733 283 MB
18 03347312d01b 740 MB
18-slim 496088b5ff70 287 MB
19 7f762ce50f2c 803 MB
19-slim 2c49447c0479 377 MB
20 24aaa2505fc5 855 MB
20-alpine cad3f87c1031 69.9 MB
20-slim 21b805e82d04 282 MB
master de2d23b1d874 794 MB
master-alpine 83db03d437ab 67.9 MB
```
`s390x`:
```
TAG IMAGE ID SIZE
17 fe358d304581 728MB
17-slim eabdc152d3ff 287MB
18 63ef88c695e2 734MB
18-slim 0c7b31edc27f 290MB
19 a5fedc3365b6 806MB
19-slim 9b01eb0db543 386MB
20 0df5ebfae4f7 859MB
20-alpine 5eaba6a4bac2 67.9MB
20-slim 8b8583668671 282MB
master 0446b22ce6ac 792MB
master-alpine be3a98bf8969 65.9MB
```
`ppc64le`:
```
TAG IMAGE ID SIZE
17 479b69dfb8e4 752MB
17-slim 2250cc40ef1a 286MB
20 3e9fb910771a 885MB
20-alpine 20fa16b73284 70.8MB
20-slim 7d451df78433 281MB
master b2676b68fc79 819MB
master-alpine 2e6b99a7ee67 68.8MB
```
