Problem: isolated Linux testing, release builds

Currently, running some of the tests can be problematic
if SIT is also being used on that same machine by the same
user as SIT will try to pick up the existing user config.

Also, building releases requires the additional setup of
musl target.

Solution: provide a Docker container with the musl build environment

.gitignore

@@ -2,3 +2,4 @@ target
 .idea
 .sit/items/*/.lock
 ._docker_osx
+._docker_linux

Makefile

@@ -1,7 +1,14 @@
 osx: target/x86_64-apple-darwin/release/sit target/x86_64-apple-darwin/release/sit-web
+linux: target/x86_64-unknown-linux-musl/release/sit target/x86_64-unknown-linux-musl/release/sit-web
 
 target/x86_64-apple-darwin/release/sit target/x86_64-apple-darwin/release/sit-web: build-tools/cross-compile-osx/Dockerfile sit-core sit-web sit
 	docker build --iidfile ._docker_osx build-tools/cross-compile-osx
 	sed -i s/sha256://g ._docker_osx
 	docker run -u `id -u`:`id -g` -v `pwd`:/sit -w /sit -t `cat ._docker_osx` sh -c "cargo build --release --target=x86_64-apple-darwin"
 	rm -f ._docker_osx
+
+target/x86_64-unknown-linux-musl/release/sit target/x86_64-unknown-linux-musl/release/sit-web: build-tools/linux-build-container/Dockerfile sit-core sit-web sit
+	docker build --iidfile ._docker_linux build-tools/linux-build-container
+	sed -i s/sha256://g ._docker_linux
+	docker run -u `id -u`:`id -g` -v `pwd`:/sit -w /sit -t `cat ._docker_linux` sh -c "cargo build --release --target=x86_64-unknown-linux-musl && strip target/x86_64-unknown-linux-musl/release/sit target/x86_64-unknown-linux-musl/release/sit-web"
+	rm -f ._docker_linux

build-tools/linux-build-container/Dockerfile

@@ -0,0 +1,3 @@
+FROM rust:1.26.2
+RUN apt-get update && apt-get install -y cmake libgit2-dev musl-tools
+RUN rustup target add x86_64-unknown-linux-musl

build-tools/linux-build-container/README.md

@@ -0,0 +1,12 @@
+This container provides a convenient way to build and test sit in isolation
+from the local environment (Linux-specific). This is particularly useful for
+running integration tests that might inadvertently read user configuration and
+fail because of that (or introduce undesired behaviour such as requiring gnupg
+signatures and triggering gpg-agent popups) ir building releases.
+
+A suggested way to run such isolated environment would be:
+
+```
+docker run -v /host/path/to/sit/repo:/sit -e CARGO_TARGET_DIR=/sit/test_target -ti <image tag>
+```
+