[docker] exit early if build fails

If the build fails, don't then attempt to run the container-which-wasn't-built

.devcontainer/build.sh

@@ -1,3 +1,9 @@
 #!/bin/sh
+set -eu
+
+# A little helper script to rebuild the generated files using a known-good
+# environment, so that we should all end up with the same results, and also
+# no need to install any dependencies on the host machine.
+
 cd $(dirname $0)/../
 docker run --rm -v ${PWD}:/app $(docker build -q -f .devcontainer/Dockerfile .)

.devcontainer/run.sh

@@ -1,3 +1,8 @@
 #!/bin/sh
+set -eu
+
+# A little helper script to run a shell in a known-good environment, so that
+# developers can easily debug issues with the build process.
+
 cd $(dirname $0)/../
 docker run --rm -ti -v ${PWD}:/app $(docker build -q . -f .devcontainer/Dockerfile) /bin/bash