Leave out of the build context what the app's .dockerignore names (closes #274)

Docker does not apply .dockerignore to a build context sent as a tar,
so upaas sent every file in the clone. The build now reads the ignore
file as docker build does, with the ignorefile reader of
moby/patternmatcher: <Dockerfile>.dockerignore next to the Dockerfile
if there is one, otherwise .dockerignore at the root. As Docker does,
the Dockerfile path is read as a path inside the clone, and when a
Dockerfile named Dockerfile is missing, a lowercase dockerfile next to
it is used instead. As the docker command line does, the Dockerfile and
.dockerignore are never left out. The ignore file is read through
os.Root. An ignore file that cannot be read, or a malformed pattern,
fails the build.

Model: opus-5-5

go.mod

@@ -15,6 +15,7 @@ require (
 	github.com/joho/godotenv v1.5.1
 	github.com/mattn/go-sqlite3 v1.14.32
 	github.com/moby/buildkit v0.16.0
+	github.com/moby/patternmatcher v0.6.0
 	github.com/oklog/ulid/v2 v2.1.1
 	github.com/prometheus/client_golang v1.23.2
 	github.com/spf13/viper v1.21.0
@@ -60,7 +61,6 @@ require (
 	github.com/klauspost/compress v1.18.2 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect
 	github.com/moby/locker v1.0.1 // indirect
-	github.com/moby/patternmatcher v0.6.0 // indirect
 	github.com/moby/sys/sequential v0.6.0 // indirect
 	github.com/moby/sys/signal v0.7.1 // indirect
 	github.com/moby/sys/user v0.4.0 // indirect