ci: build and publish the image on every push to main

The registry account available on the testnet hosts is pull-only (403 on a
blob upload even for an existing repository), so publishing goes the same way
every other BitDeals image does: the Gitea runner builds and pushes with the
CI credentials. Tags mirror the family — an immutable <version>.<sha7>, a
moving <version>, and :latest for compose and Watchtower.

The version is read out of the Dockerfile ARG rather than repeated here: a tag
that can disagree with the code inside is worse than no tag.

.gitea/workflows/build.yaml

@@ -0,0 +1,48 @@
+name: Build docker image and push to registry.bitdeals.org
+run-name: docker build and docker push
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      COMMIT: ${{ gitea.sha }}
+      REGISTRY: 10.0.3.111:5000
+      IMAGE: electrumx
+      USER: ${{ secrets.DOCKER_USERNAME }}
+      PASS: ${{ secrets.DOCKER_PASSWORD }}
+    steps:
+      - name: Checkout repository code
+        uses: actions/checkout@v6
+
+      # The version tag comes from the Dockerfile rather than from a second
+      # copy here: the ARG is what the build actually installs, and a tag that
+      # can disagree with the code inside is worse than no tag.
+      - name: Resolve the ElectrumX version from the Dockerfile
+        run: |
+          v=$(sed -n 's/^ARG ELECTRUMX_VERSION=//p' docker/Dockerfile)
+          test -n "$v" || { echo "ELECTRUMX_VERSION not found in docker/Dockerfile" >&2; exit 1; }
+          echo "VERSION=$v" >> "$GITHUB_ENV"
+
+      # Three tags: the immutable one to deploy by, the moving version tag for
+      # people reading `docker images`, and :latest for Watchtower and compose.
+      - name: Build image
+        run: |
+          docker build . \
+            --file docker/Dockerfile \
+            --label "git-commit=$COMMIT" \
+            --tag "${REGISTRY}/${IMAGE}:${VERSION}.${COMMIT::7}" \
+            --tag "${REGISTRY}/${IMAGE}:${VERSION}" \
+            --tag "${REGISTRY}/${IMAGE}:latest"
+
+      - name: Docker login
+        run: docker login --username "$USER" --password "$PASS" "$REGISTRY"
+
+      - name: Push images
+        run: |
+          docker push "${REGISTRY}/${IMAGE}:${VERSION}.${COMMIT::7}"
+          docker push "${REGISTRY}/${IMAGE}:${VERSION}"
+          docker push "${REGISTRY}/${IMAGE}:latest"

README.md

@@ -60,6 +60,11 @@ Anything after the image name is passed on to `electrumx_server`.
 
 ## build and publish
 
+A push to `main` builds and publishes the image (`.gitea/workflows/build.yaml`),
+tagging it three ways: `<version>.<sha7>` to deploy by, `<version>` to read, and
+`latest` for compose and Watchtower. By hand, when the registry credentials are
+at hand:
+
 ```sh
 docker build . --file docker/Dockerfile --tag registry.bitdeals.org/electrumx
 docker push registry.bitdeals.org/electrumx

README.ru-RU.md

@@ -60,6 +60,11 @@ docker run -d \
 
 ## сборка и публикация
 
+Пуш в `main` собирает и публикует образ (`.gitea/workflows/build.yaml`) с тремя
+тегами: `<версия>.<sha7>` — для развёртывания, `<версия>` — для чтения глазами,
+`latest` — для compose и Watchtower. Вручную, если под рукой есть учётные данные
+registry:
+
 ```sh
 docker build . --file docker/Dockerfile --tag registry.bitdeals.org/electrumx
 docker push registry.bitdeals.org/electrumx