This PR adds support for building Gridsync inside a CentOS 7-based Docker/OCI container (via the make in-container make target), thereby facilitating the building and distribution of backward-compatible Gridsync binaries on GNU/Linux hosts (and addressing #328)

In addition, this PR includes a some minor updates to the build-process which result in Gridsync AppImages building reproducibly/deterministically on the same host; running make clean in-container multiple times in a row on the same machine will result in bit-for-bit identical outputs (given the same inputs/source-code). Some variation has been observed across some hosts, however (for example, between older and newer versions of Ubuntu), suggesting that some sources of indeterminacy still remain, but investigating -- and addressing -- these is outside the scope of this PR.