Add i386 and openjdk dockerfiles

In the i386 image, `flags='arch=i386' ./test/ci.sh` passes.
In the openjdk image, the following all pass:
* `flags="openjdk=${JAVA_HOME}" ./test/ci.sh`
* `flags="openjdk=${JAVA_HOME} openjdk-src=/var/openjdk-src" ./test/ci.sh`
* `flags="openjdk=${JAVA_HOME} openjdk-src=/var/openjdk-src arch=i386" ./test/ci.sh`

With the caveat that the process=interpret test will fail on the latter two if you're on a VM.  That's a separate issue.

The conspicuously absent combination above is of course i386-openjdk.  This will require also including a 32-bit VM in the image.

I think the next step will be to build a "mega" image that includes as many possible build dependencies as possible (32/64 openjdk support, openjdk-src, windows & arm cross-compilers, lzma, android, ...).  I'm not sure how which (if any) of the individual or base images I'm building will be useful.

EDIT: These images are pushed as `joshuawarner32/avian-build-i386` and `joshuawarner32/avian-build-openjdk`, respectively.