properly check Docker version

[anarcat]

dpkg --compare-versions is debian-specific, but we're already debian-specific here.

Closes: #869

[anarcat]

this is an alternative fix for #869 that possibly addresses the fundamental issue a little better. i think it might still be worth having a way to skip this mechanism in any case.

[anarcat]

i will note that the conflict here was due to a rewrite of that version comparison to this function:

version_gt() (
    set +x

    yy_a="$(echo "$1" | cut -d'.' -f1)"
    yy_b="$(echo "$2" | cut -d'.' -f1)"
    if [ "$yy_a" -lt "$yy_b" ]; then
        return 1
    fi
    if [ "$yy_a" -gt "$yy_b" ]; then
        return 0
    fi
    mm_a="$(echo "$1" | cut -d'.' -f2)"
    mm_b="$(echo "$2" | cut -d'.' -f2)"
    mm_a="${mm_a#0}"
    mm_b="${mm_b#0}"
    if [ "${mm_a:-0}" -lt "${mm_b:-0}" ]; then
        return 1
    fi
    if [ "${mm_a:-0}" -gt "${mm_b:-0}" ]; then
        return 0
    fi

    bb_a="$(echo "$1" | cut -d'.' -f3)"
    bb_b="$(echo "$2" | cut -d'.' -f3)"
    bb_a="${bb_a#0}"
    bb_b="${bb_b#0}"
    if [ "${bb_a:-0}" -lt "${bb_b:-0}" ]; then
        return 1
    fi
    if [ "${bb_a:-0}" -gt "${bb_b:-0}" ]; then
        return 0
    fi

    return 1
)

which... i mean. i am having a hard time putting this nicely, but let's just say this could be slightly more readable and less buggy, to say the least. that function is 35 lines long, compared to my one-line patch, and doesn't actually work for debian versions:

helpers.sh: line 190: [: 24+dfsg1: integer expression expected

so i've rebased this and urge maintainers here to take another look at this merge request instead of reinventing the wheel like this. i haven't actually ripped out version_gt but i would strongly recommend getting rid of that code.