Allows running spec within Docker container

[luislavena]

crystal spec commands now supports JUnit XML output, which requires libxml2 to be installed.

With current defaults, the command fails due missing libxml2-dev dependencies in the image.

This change ensures libxml2-dev is installed along the other dependencies so can be used for both building executables and running specs.

Without this change, attempt to use official docker image fails with the following:

# crystal spec
/usr/bin/ld: cannot find -lxml2
collect2: error: ld returned 1 exit status
Error: execution of command failed with code: 1: `cc -o "/root/.cache/crystal/crystal-run-spec.tmp" "${@}"  -rdynamic  -lyaml -lxml2 -lpcre -lm -lgc -lpthread /opt/crystal/src/ext/libcrystal.a -levent -lrt -ldl -L/usr/lib -L/usr/local/lib`

But with this change:

# crystal spec
..............................................

Finished in 2.21 milliseconds
46 examples, 0 failures, 0 errors, 0 pending

I'm still not sure if including xml is desired for the release container, but considering might others need XML support during compilation too perhaps is not a bad idea?

Thank you ❤️ ❤️ ❤️ ❤️

[asterite]

Thank you for this!! ❤️

I think we should include all dependencies that some part of the standard library depends on. So far I think it's libxml2 and libyaml.

And I'd do it to for the release container.

[luislavena]

@asterite I had libyaml-dev in my original change but decided only to include libxml2-dev since spec was failing.

The change is only for Dockerfile.release which I believe the one used to push to Docker Hub?

I'm adding now the change for yaml 😄

[asterite]

Thank you!! ❤️