statically allocate app container IPs

We removed httpPort with the assumption that docker allocated IPs
and kept them as long as the container is around. This turned out
to be not true because the IP changes on even container restart.

So we now allocate IPs statically. The iprange makes sure we don't
overlap with addons and other CI app or JupyterHub apps.

https://github.com/moby/moby/issues/6743
https://github.com/moby/moby/pull/19001

src/iputils.js

@@ -0,0 +1,35 @@
+'use strict';
+
+exports = module.exports = {
+    ipFromInt,
+    intFromIp
+};
+
+const assert = require('assert');
+
+function intFromIp(address) {
+    assert.strictEqual(typeof address, 'string');
+
+    const parts = address.split('.');
+
+    if (parts.length !== 4) return null;
+
+    return (parseInt(parts[0], 10) << (8*3)) & 0xFF000000 |
+           (parseInt(parts[1], 10) << (8*2)) & 0x00FF0000 |
+           (parseInt(parts[2], 10) << (8*1)) & 0x0000FF00 |
+           (parseInt(parts[3], 10) << (8*0)) & 0x000000FF;
+}
+
+function ipFromInt(input) {
+    assert.strictEqual(typeof input, 'number');
+
+    let output = [];
+
+    for (let i = 3; i >= 0; --i) {
+        const octet = (input >> (i*8)) & 0x000000FF;
+        output.push(octet);
+    }
+
+    return output.join('.');
+}
+