Make sure we pick the exact same compile environment for Tor Browser builds

When creating a container image at time t and creating a container image at time t1 it is not guaranteed that we include the same compiler versions etc.

This has led to mar executables being different, for instance (see: comment:52:ticket:26475): GCC version was in both cases 4.9.2 but in one of them contained an updated resulting in GCC: (Debian 4.9.2-10+deb8u1) 4.9.2 as a version string instead of GCC: (Debian 4.9.2-10) 4.9.2.

We could fix that by making sure we compile our own GCC for this case but there might be other packages lurking with the same trouble potential. We should make sure instead that everyone creating and using a container image is getting/having the same available.