dockerize: Support complex dependency install lines.
dockerize: Support complex dependency install lines.

Wrap the DOCKER_DEPS_CMD with `sh -c` instead of directly executing it,
to allow running multiple commands subshells there.
(This is needed for Debian/Ubuntu that don't have a single
"update repos and install stuff" command).

I tried passing DOCKER_DEPS_CMD="sh -c '....'" but some kind of crazy
double-quoting rule kicked in and confused everything.

Shell is unforgiving!

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