Stamp the git tag or short commit in a plain docker build (closes #46)

A plain `docker build .` stamped `unknown` into the page footer:
.dockerignore left out .git, and the Dockerfile built without the -X
flags. The context now carries .git without .git/config, which can hold
a credential. The build stage takes the VERSION build argument when one
is given, otherwise `git describe --tags --always`, fails the build if
.git is present and no version comes out, and stamps it together with
the full commit the footer links to. `make build` now uses
`git describe` as well, and script/docker is the current shared copy.

Model: opus-5-5

Makefile

@@ -2,7 +2,7 @@ export DEBUG = routewatch
 
 # Git revision for version embedding
 GIT_REVISION := $(shell git rev-parse HEAD 2>/dev/null || echo "unknown")
-GIT_REVISION_SHORT := $(shell git rev-parse --short HEAD 2>/dev/null || echo "unknown")
+GIT_REVISION_SHORT := $(shell git describe --tags --always 2>/dev/null || echo "unknown")
 VERSION_PKG := git.eeqj.de/sneak/routewatch/internal/version
 LDFLAGS := -X $(VERSION_PKG).GitRevision=$(GIT_REVISION) -X $(VERSION_PKG).GitRevisionShort=$(GIT_REVISION_SHORT)